If you're trying to get Character Select working specifically, don't put it in DynOS. Character Select is a Lua mod and should be in the mods folder.
Also, if you've been swapping Character Select versions around, completely remove the old Character Select files first before installing the new one. A bunch of people have run into script errors because old files were left behind.
Another thing: what version of Character Select are you actually using? If you're on CoopDX 1.5.1 and grabbed the latest Character Select release, there's a chance you've got a version mismatch. Character Select gets updated pretty often and some releases have had compatibility issues.
I'd also check whether the mod is showing up in the mods list at all. If Character Select itself isn't appearing, that's an install problem. If it appears but won't open, that's usually a script error or version mismatch.
Ex.
Mod showing up in game
Error on 1.5.1