When installing jBPM 5 using the installer, everything worked fine, until I tried opening Eclipse directly, using the .exe. I did this so I could write my own programs without having to launch from the command prompt using "ant start.demo" every time. Also, I didn't want to launch unnecessary content.
Launching Eclipse like this removed the jBPM runtime from settings, and so any import org.drools... line didn't work.
I fixed this by going to the menu Window, then Preferences, treeing jBPM, and adding a new runtime that pointed to the runtime folder within the jBPM folder the installer extracted to. This solved it.