That was driving me crazy as well, having World Edit working fine but no CUI. Finally after a lot of searching, I found a litemod version among the many posts in the Minecraft forum
thread [
direct download link as posted on the thread]. It's also supposedly included in the Voxel Modpack, so you could possibly get it that way too.
Just drop it into the mods folder in your modpack install and it works fine, at least for me. I'm running Ultimate which has liteloader, so you would need a pack with liteloader as well. Of course you would also need to install world edit to your server or for SSP, but I assume you have done so already (if not the previous post has instructions).
Note that although it is a litemod, it won't show up in the "Lite Mods" in the main menu after you start the game. I just went on to the server and tested it, and it worked.