Word clock and MMC are two different things. You seem to be talking MMC (Midi Machine control) as near as I can tell. You need a midi breakout cable on both the HDR and d8b with midi cables in between, in to out, out to in, to make the loop complete so that your HDR will follow MMC commands.
Here is a link to an Ebay auction with the breakout cables I speak of. You may have these in place, just couldn't tell for sure by your post.
2pcs MIDI Breakout Cable for Mackie HDR / MDR / D8B | eBay
http://r.ebay.com/JdAZEgGet these in place with two midi cables and the right boxes checked on the HDR and d8b and you should be off to the races.