Can you try connecting to the stream from a PC (windows: winamp, linux: xmms, mac: dunno)? If you can't do it from a PC, it's not a 770 issue.
You could also try dropping the password, just to test to see if that's the issue.

FWIW, I don't have any trouble streaming to the 770. general form of the URL should be http://your.slimserver.ip.address:9000/stream.mp3

cheers,
#!/ben