How to get a Meridian 518 to pass a DD 5.1 signal?

I have a Meridian 518 / 562 v1 / 565 (no Z3/DTS upgrade) processor in my surround system. The 518 has been updated by Meridian with the latest hardware and software (I sent the 518 to Meridian in Atlanta for the upgrade last summer) but will not pass a Dolby Digital 5.1 signal as the upgrade is supposed to allow. It will pass a PCM bit stream. And the 565 will accept a DD 5.1 signal directly. Any suggestions? Is there a secret to the set-up that I am missing?

Before I send the 518 back to Meridian, I would like to make certain that I have tried all the configuration and set up options.

I had a 518 going into a 565,but mine had the dts 7.1 installed. There is a Meridian user site called something like "hitchhiker guide to Meridian" That may not be the exact add, but it is something like that. I sold my 518 because of space consideration;and I didn't think it improved things all that much. After all it is just a digital filter device and as I remember you couldn't do input switching. It's been a while I hope the switching thing wasn't my error on hookup.
All kinds of neat stuff at that website, including at least 3 threads on my problem. Much appreciated!!