I installed the latest vm (3.10-4 #2) from squeakvm.org web site as my previous vm was apparently without ffi (or could it be the plugin compiled in the VM?).
SmalltalkImage current listLoadedModules
In pharo, I would install FFI from ScriptLoader. Just like this:
ScriptLoader loadFFI.
See http://code.google.com/p/pharo/wiki/CodeSnippets