Play with a keyboard instrument
If your keyboard has a synthesizer on board that is GM compatible (can play 128 different instruments) then you can use it in One Man Band. If not then the best choice is to install the Roland Sound Canvas app for iOS and select that in Settings/MIDI Out.
A free alternative is to use the internal soundfont player. In Settings select MIDI Out is Soundfont player. Touch the "No soundfont selected" button and then the "Download Soundfont" button. You will go to the soundfont page in Safari. Download a soundfont and select "Copy to One Man Band". You will return to the soundfont list in OMB, touch the downloaded soundfont to activate it.
To connect the iPad to the keyboard you need an adapter that connects to an USB plug. That is the Apple camera connection kit. You just need the one where a USB can plug in, not the one for the SD cards. Make sure that you get the one that fits in your iPad/iPhone/iPod because the older models have a wide 30 pins connector, while the new models have the small lightning connector. You also need a MIDI Interface cable. They come in two types and both have a normal USB plug on one side. What's on the other side depends on your keyboard. Older keyboard models need two round DIN plugs, newer models need a square USB plug (AB cable or printer cable).
When the cables are connected you must select the Midi Ports in the Settings panel in One Man Band. You may need to switch your keyboard to "computer mode" although that is often done automatically when you plug in the MIDI cable. Switch the keyboard to 'local off", it shouldn't produce any sound by itself when you hit the keys, it should only play the notes that come thru MIDI from the connected iPad. You can find information on how to to that in your keyboard manual.
Old 30
pins adapter
New lightning adapter
MIDI
cable
USB AB cable