I use a plugin called "creeper control". It's really simple, and it works for me on 2.1.1 servers
http://dev.bukkit.org/server-mods/creepercontrol/
I'm getting the same message, but I'm running a Tekkit server on my own computer and I made sure I was playing Tekkit and not Technic Pack... tried to run a search and couldn't find the problem...
Edit... got it working, didn't realize I had to manually update tekkit to 2.1