OK, I should be a little more specific. Reinstall whatever flavor of FTB you are using (I'm using Mindcrack) - That's your /coremod, /mods and /config folders and all files within. In your root folder of the server you should have a /jar folder. That's where you'll place the jar from the MCPC+ zip. Rename it ( I named it ftbserver.jar). Reboot your server, and then you should be able to place plugins in your /plugins folder.