You don't even need to hit return, actually--just type/paste and wait.
Works perfectly for me--takes on the order of one second to translate a small block of text I paste in in Japanese. It does sometimes seem to run slower--I assume when Systran is bogged down--but at least late at night on a Friday it's peppy as anything.
If you enter something, wait for a while, and nothing happens, you might try coming back after a bit--the translation service does flake out on occasion, but it always comes back for me. I have heard stories of it dying more permanently, but apparently it can be brought back is deleting its pref file (widget-com.apple.widget.translation.plist).
Try that, then reload it (click on it, then hit command-R).