The root of the problem is that you have not installed the modmerge tool to your SoD game that is needed to mod it. As the dialog between the .zip and original game is a different one.
To fix this, uninstall your mods, install the modmerge, then reinstall your mods and start a new game(as in, don't load a save as those are not compatible with the install you have then done).
I tried it and it works fine! There's only one little problem now: in the character properties window some proficiency text is simply missing, only the ++ is visible. I use Baldurs Gate proficiencies with weapon styles from the tweaks anthology. Any idea?
Thanks for the help anyway, my game is now completely playable!