The easiest way to get the mod to show in game is to navigate to your Documents/MyGames/GC2Twilight( or Ultimate Ed.'s equivalent) and find the Prefs.ini file . Open it with notepad and find the line that looks something like this:

ModsDir=D:\Program Files (x86)\Stardock Games\GalCiv2\DarkAvatar\Mods\High-Resolution Graphics Mod v1-2

Yours may be a little different being Ultimate edition, but close enough to the example.

Just rewrite it to point to the HiRes mod's folder and save. 

One thing to note on mine the HiRes's installer seemed to put the mod's folder in an old GC2 folder structure. Something like Stardock/TotalGaming/GC2.... or some such. I manually copied it back into the proper location in the games Twilight/Mods directory, but that is up to you since I believe with the above you can have the game look about anywhere for it anyways.

One of the promotion threads for it(the one that should be deleted) has a link to a defunct GalCiv2Guide site. There is another post somewhere with the Imod link. Seems maybe a certain AV is giving false positives on it. I have DLd both 1.2 and 2.0 versions multiple times from Imod, scanned them with several reputable AV proggies and anti Adware/spyware and never had it find anything.