In the physical connection is described very well in Cisco DPA 7630/7610 Voice Mail
Gateways Administration Guide. I installed recentlly DPA 7610 for Octel 200 system without any problem. Connection to PBX is correspond one to one, for Octel 250 you need to look carefully in guide how to connect.
Be carefull with port in DPA. You need to choose hybrid mode; in this mode first 8 ports of DPA could be used for in/out messages, next 8 ports for Octel MWI and the last 8 port for PBX Meridian MWI.