Plugging a MIDI Device in a MacBook Pro What options are there for plugging a MIDI-device that offers only a standard DIN-5 MIDI connector into a MacBook Pro?

The marketing print on the box of my Rock Band 3 Wireless Keyboard talks about plugging the keyboard into a computer for use as a general MIDI instrument when not playing Rock Band, but there's no cables inside the box to accomplish this, and again, the only port it offers is a DIN-5 port.
Simple and cheap: USB to MIDI connector. If you need to plug in more than one MIDI device there are dedicated MIDI interfaces. Additionally, many A/D audio interfaces - that you might also use to record input from analog devices like guitars or microphones - include MIDI ins/outs.

Without more information about your specific needs I can't say the products I linked to are the best choice, but they are representative of the vast array of options for plugging a MIDI device into your MacBook Pro.