Hi Sparoz,
I'll assume that you'd like to have the centre speaker as master.
There are three settings you need to make:
1) Set the centre speaker as master using the MSR, so it is the only speaker in your system with a red LED on it. (When in normal use, the red led also comes on slave speakers while configuring them)
2) Set the Centre as master in the Meridian Configuration program (so that has a red 'led' next to it) and upload to the 861V8.
3) The speakerlink cable from the centre speaker MUST go into the centre socket on the 861v8 that has a green stripe next to it.
I'm guessing 3) is the problem here.
Rick