I just installed a DeckLink HD 3D+ on one of our Windows 7 Systems and experienced the same problem. The card worked with Nuke, After Effects, Resolve and Premiere but not Avid Media Composer. After going through the forums and looking at this thread. I found this temporary solution.
I installed the 9.6.1 drivers. Then I downloaded the 9.5.3 drivers from the Black Magic Website (
http://www.blackmagicdesign.com/support/)
Then I extracted the files from the DesktopVideo_9.5.3.msi file using this comand:
msiexec /a PathToMSIFile /qb TARGETDIR=DirectoryToExtractTo
Once extracted, I went into the "\Blackmagic Desktop Video\Avid Support" folder from the extracted directory and manually copied the "OpenIO_Blackmagic.acf" file into my Avid's "AVX2_Plug-Ins" folder. So far it works fine but haven't had a chance to test it a lot.
I extracted files from the 9.6.1 drivers and the avid plugin was not included in the release, whether this was an oversight, I don't know. But try it and see if that works for you.
-Alexander