There are a few things you can try, here is what I would try if I were in your shoes:
Make sure that the versions of your mods and minecraft match up - are you playing on Minecraft v1.5.0 but using World Edit 1.5.1?
If you are using the proper version of everything, try these commands and seeing if you get the same error (please don't be surprised if any of these spit out an unknown command error to you) :
Where the 50 represents the radius around you, meaning any mobs within a 50 block radius will be killed.
If that still doesn't work, try forceupdating your minecraft, then installing SinglePlayerCommands on your clean minecraft.jar.
There might be other mods, such as ones that add in new blocks, or even mobs, which might be interfering with you. So make sure to check (that if you are using these other mods) that you have compatible versions.