There's the problem there: You selected the Doom 3 folder itself when you need to actually go through the common folder like so:
C:\Program Files (x86)\Steam\SteamApps\common
Once you do that, you should have something like this before the installation progress starts:
C:\Program Files (x86)\Steam\SteamApps\common\Doom3
Only then will you be able to enjoy the mod in all it's glory.
NemesisTrestkon
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