My free DLC program refers. I received an e-mail with order ID number but where to from here?
If I go my my accounts it displays the DLC program but I see no download option.

In Account -> Games -> The Witcher 3: Wild Hunt. The DLCs should be listed down at the bottom.

For Galaxy it's Library -> The Witcher 3: Wild Hunt -> More -> Configure. From there you'll be able to check the boxes to install the individual DLCs.

Here you go:

Method 1 - Download using the Galaxy Client:
1. First when you look at the game page you will see that there are DLCs on the left. The bottom one is the one you are looking for.
Go ahead and buy it, don't worry no payment is required (mine already says owned because I did this step).
2. Now to activate the DLCs press the 'more' tab next to the play button and go to configure.
After you have entered the configure section go ahead and mark your selected DLCs.
3. Enjoy your free content and don't forget to drop the developers a nice word.
Have fun and may your hunt be successful.

Method 2 - Download Manually:
1. First thing first you must go to this page and buy the free DLCs Program.
Go ahead and buy it, don't worry no payment is required.
2. After you have finished the purchase go to your account page.
3. Press the Witcher 3 icon or "folder" as I like to call it.
If you don't see the Witcher 3 "folder" you probably bought a retail copy. If that is the case, you must first redeem the code found inside the retail box into your GOG.com account.
4. After the drop down menu opens select the DLCs you wish to download under "DLC Downloads".
5. Save the .exe files and install them manually.
6. Enjoy your free content and don't forget to drop the developers a nice word.
Have fun and may your hunt be successful.

Source: http://www.gog.com/forum/the_witcher_3_wild_hunt/guide_free_dlcs_activasion/post1

You need to own The Witcher 3: Wild Hunt, to have access to the DLC files.

Then just follow the steps above.

The games tab gives me 26 pages of games. Do I have to go searching for Wild Hunt?
The Witcher 2 is on page 1 Are you referring to the list of downloads where the DLC is broken down to about 20 items, a few new quests, one to change Yen's appearance etc.

Here is the list of the DLCs:

1. Temerian Armor Set
2. Beard and Hairstyle Set for Geralt
3. New Quest - Contract: Missing Miners
4. Alternative Look for Yennefer
5. Nilfgaardian Armor Set
6. Crossbow Set - Three New Crossbows
7. New Quest: Fool's Gold
8. Neutral Gwent Card Set: Ballad Heroes
9. New Quest - Scavenger Hunt: Wolf School Gear
10. Triss Alternative Costume
11. New Quest - Contract: Skellige’s Most Wanted
12. Skellige Armor Set
13. New Quest: Where the Cat and the Wolf Play
14. Ciri Alternative Outfit
15. New Finisher Animations
16. New Game + You asked for the DLCs and I tried to help you. You need to explain me your problem a bit more because I don't understand what your problem is exactly?

There is not 1 downloadable file with all DLCs. You have to install them one by one.

Not the 'Games' tab, your 'Account' tab. Press on your 'Account' tab, and only the games you have purchased will be displayed. Click on 'The Witcher 3' in your account and you will see the downloads right in front of you. It couldn't be simpler.

This is what has lead to my confusion.
Gog sent me an e-mail confirming order of DLC program with ID number.
I have downloaded four new quest downloads but was not aware that the list of 20
was related to the said DLC program and nowhere was this ID number required.

Thanks for this. Can anyone suggest which ones are worth downloading besides the new quests?

A completely subjective question, for which you will get differing answers from different people. All of them are free, and none of them are large download -- the entirety is only around 500MB -- so why not download them all?

I have just done the DLC Fool's Gold quest. But to gain only 2 experience points make this quest pointless.
Unfortunately all low level side quests seem to give virtually no experience points.

That's because the quests don't level with the character. Level 10 quests are very easy to do for a level 30 character, so the reward is very small. Conversely, taking on quest above your level will reward you with greater amounts of XP.

I see. That makes sense.

OK. I downloaded and installed everything but cannot find the merchant named Bram at White Orchard
nor a blacksmith at Kaer Trolda, only the armorer. (there is a blacksmith sitting smoking a pipe but cannot interact with him and the map shows no blacksmith symbol here)
I also cannot find the quartermaster in Crows perch.
Finally, to change Yen's appearance it says to go to the Game Menu but where is this menu?
Please, can anyone assist?

Bram is the merchant that you can save from the Griffin at the very start of the game, with Vessemir. You will find him just outside of the inn.

If the blacksmith in Kaer Trolde is sitting smoking his pipe, he is not available (on a break). Come back later.

The quartermaster in Crows Perch is to the left of the archway (as you go in) where the master armour (Yoana) is.

Cosmetic DLCs can be activated/deactivated from the main menu. You must quit the game to the main menu. It's in 'Downloadable Content'.