I believe there is a problem with the "pre-converted" (from MOM 2005) management pack that is offered for BizTalk 2006 on the SCOM 2007 management pack download page.

I believe the conversion started with the BizTalk 2006 (flat) MOM 2005 management pack, rather than the BizTalk 2006 R2 version.  The latter is available here.

Assuming you have already installed the MOM 2005 Backward Compatibility Management Pack, version 6.0.5000.12, you should be able to import the 2006 R2 version (via the Operations Manager Migration Wizard.)

The reason this is important is because the computer attribute that the (older) management pack is looking for includes:

BizTalk 2006 R2 has a version stamp of 3.6.1404.0.

(I believe other folks have run into this issue as well.)

Update: Because the import/conversion process is a bit of a hassle, I thought I’d post the SCOM-converted files here for download.  These were produced by starting with the R2 AKM files, converting to xml via mp2xml.exe (in the MOM 2005 resource kit), and then finally apply MPConvert.exe from the SCOM tools.