Thanks to the guys on the GHCJS IRC Channel I got the answer:
foreign import javascript safe "$1.subscribe($2)"
rx_subscribe :: Observable a -> JSFun (a -> IO()) -> IO Disposable
subscribe :: FromJSRef a => (a -> IO()) -> Observable a -> IO Disposable
subscribe f xs = syncCallback1 True True f' >>= rx_subscribe xs
where f' x = fromJSRef x >>= f . fromJust
Thank You