FTB runs on forge. Plugins need bukkit to run, but cannot be run on forge. So you need a bukkit-forge jar, which will let the modpack run through the forge side, while allowing plugins to run through bukkit, all in 1 jar. This is called MCPC+, which you can find here:
http://ci.md-5.net/job/MCPC-Plus/
Replace the ftb.jar with the mcpc.jar, run the server, and now you should have a plugins folder. Install plugins normally by drop and dragging.
Tada.