How to connect my keyboard so I can use it when recording software instruments

Ok so I have logic express 9.1.7... I play guitar so I have been using it to record real instruments for a few months now, but I want to expand my recording ability and start using the software instruments that logic provides because they are great. I have a YAMAHA DGX-505 portable grand piano that I have running into my steinberg sound card which then runs into my mixer and finally into my computer. Everything works great, but when I try using the keyboard as a MIDI device (or whatever it is that allows me to use it as a controller for the software instrument) no sound is heard. I want to be able to use this feature without having to use the keyboard on my computer because it limits the range I can play. So any help would be GREATLY appreciated