Occasionally, WorldEditCUI will release an update in between Minecraft release cycles. This means that class files will need to be rewritten, replaced, and it will often break your client.
To update, you have no other options but to reinstall everything, including all of your other mods. To do this, open the Minecraft launcher, hit the "Options" button, and click "Force Update". This will redownload the minecraft.jar. From there, follow the instructions in INSTALL.txt.
If anything breaks due to this update, you should start over with a clean minecraft.jar before seeking help.