Man there's tons of software out there. Fruityloops is one, and there are many others. Do a search on the web. What you'll need for the 550 to connect is a Midi to USB converter. It's a box that has standard midi cables to plug into your 550 and the other end is a USB adapter to go into the USB port of your PC. They're around $30-$40 in most places.

Squeak
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.