MIDI/Digital Guitars transmit MIDI over 6 different channels, as opposed to a MIDI keyboard that transmits over a singe channel, so you can't plug it into an external hardware synth straight out of the box with a MIDI cable. To play the sounds on your external synth/keyboard with a MIDI guitar, get Ableton Live and run the guitar into Ableton and the MIDI out into your synth using a USB/Firewire MIDI interface.