Importing TestGen questions into D2L isn't as straight-forward as one would have hoped.  After some digging around and a little experimentation, here's the process that worked for me:

1. Open the .tst file in TestGen (these instructions are for version 7 of TestGen).
2. File > Export
3. Select "Blackboard 5.x, 6.x, 7.x" under "Export Exam To:" and click "OK".
4. Specify both a last name and a question pool name and click "OK".
5. Specify the destination folder, name the file, and click "Save".
6. In Windows Explorer, navigate to that destination folder and unzip the .zip file that TestGen exported.
7. Edit the imsmanifest.xml file in  your text editor of choice.
8. Check for any <file> nodes in the <resources> node, such as for images.  I suspect those require special treatment, though I didn't test this out (instead, I found that an emdash had been converted into a graphic by TestGen, so I converted it to an — SGML character).
9. Delete the contents of the file and replace it with such:
   <?xml version='1.0' encoding='utf-8'?>
<manifest identifier='man01'>
<organization default='toc01'>
<tableofcontents identifier='toc01'/>
</organization>
<resources>
<resource baseurl='__FileNameOfDatFileHere__.dat' identifier='__FileNameOfDatFileHere__.dat' type='assessment/x-bb-pool' file='__FileNameOfDatFileHere__.dat'/>
</resources>
</manifest> 
10. Replace __FileNameOfDatFileHere__ with the name of the .dat file found in the .zip file.
11. Re-zip the two file (imsmanifest.xml and __FileNameOfDatFileHere__.dat) into a single .zip file.
12. In D2L, go to "Edit Course".
13. Go to "Import/Export/Copy Components".
14. Select "Import Components" > "from a File" and choose the .zip file, and upload it.
15. Click "Next".
16. Click "Next" again.
17. Specify a location to import to (if you wish) and check the box "Question Library".
18. Click "Next".
19. Confirm import settings and click "Next".
20. Once the import process is complete, click "Next" once more.
21. Now if you click on "Quizzes" and click "Question Library", you should see your questions (albeit uncategorized, thus I recommend only importing one TestGen file at a time, then selecting all questions and moving them to a suitable category).

[hat tip]