You don't need the mod manager. Just follow the instructions, that are simple :
(Quote from [eMOD] LargeUI mod 0.7 for FE 1.20) Manual Installation: Open the file and extract the files into your installation directory in the /Screens/ folder.
The installation directory is \Program Files\Stardock\Fallen Enchantress\Screens\ if you are'nt a steam user. If you are, then i cannot help you.
This is an excellent mod, for anyone playing on a 26"" monitor. But it doesn't add any slot in the action bar, saddly.
It is absolutly necessary to get more action slots, I agree with you.
As usual, sorry for my poor english 