Are you referring to the iSeries 124i system? There's a DDK connector on the CPU for MOH. It's the one closest to the corner of the card in the block of 4 DDK connectors.

There's also an "INT/EXT" jumper you have to change to select internal MOH, or an external source (I believe it's labeled on the circuit board).

I think there's also some programming you have to do to select MOH from the CPU, or a 4ACI interface, but it may default to the CPU.