GarageBand iOS on iPad Air, MIDI cable, Roland/Wurlitzer: How do I make something out of this combo?

I just got a 3.5mm to MIDI adapter so I can use it with my Roland EXR-3 Interactive Arranger or use it with my Wurlitzer Omni 3600, which both have in and out MIDI ports. I just want to use the cable, and involve Garageband in the use of it. How do I either: - Make the sounds from Garageband come out of the keyboard's speakers (either using the keys on the keyboard, or just playing the virtual keys on the screen), or - Record sounds from my keyboards into a Garageband track?