I don't have GC2U...but i doubt the installation principles are any different than what's used with expansions;
1) Create a ****NEW**** folder within the MODS directory.
2) Dump all modded files in it, as per the actual structure used or instructions given by the modder.
3) Start GC2U
4) Go to the options button found in the Main opening screen menu.
5) You're already into the Game tab options (second column on that screen, first group of items, top of that section). Click CHANGE.
6) Click to USE any of the installed mods (goes by the name you gave to the new folder) from its selection line in the pop-up screen listing the current directory structure. Click DONE. Not default, not cancel, DONE.
7) Tick the USE MODS box with a X.
8) Exit GC2U.
9) Re-start... your mod is active.
Assuming GC2U doesn't have the flawed BrowseWnd.dxpack introduced by the TwilightArnor expansion and kept the valid file from DreadLords... the selection lines should show up the names of folders that you created in step 1.