The problem is that when using gog installer with cedega clicking on installer buttons with mouse cursor doesn't seem to work. However, using TAB to select the right button and then hitting ENTER does work.
Am I the only one experiencing this? If no, maybe gog tema could fix the issue? :)

Odd, that issue doesn't happen with Wine or CrossOver. It might have something to do with how Cedega implements mouse input, though I was under the impression they were all basically still the same in the respect. In Wine, there is an option to disable window manager control of windows created by Wine which can sometimes lead to keyboard or mouse input issues. Does Cedega have an option like that and have you tried changing it?

Nope, at least I can't find anything like that.

I had a similar thing happen with the Gogdownloader using the linux Adobe Air client. Might be something with the way Gog implement their buttons?
Though it wasn't 100% reproducible.

Yeah, I'm a Wine person myself. I have a Linux box hooked up to my HDTV (Fallout 2 nights on the big screen, woot woot!), and Wine works swimmingly with my games. I dunno, try Wine/CrossOver rather than Cedega. After all, the best way to learn is to do it yourself.