I'm using the MPD232 and I made a short test with a screenshot included, but it should also work with MPD218:
You can see my settings: the record and monitor are ON, Out: Impact, In: MPD 232, CH 10 in my case.
If this setings are ok just press RECORD.
If you want to control the Akai with it's transport keys (STOP/PLAY/REC) you should use the MIDI CC protocol (see also MIDI Learn in S1: use on/off buttons and assign a start/stop/record command)
Hope this help's