Hi Nicole,
If you're running 1.8.3, you need to visit the Certificate page at http://moodle.org/mod/data/view.php?d=13&rid=683 and be sure to download the one that says it's for Moodle 1.8.
Once you have that set up, you can base the issuing of a certificate on a particular grade reached on your assignment/post test (be sure this test is set up first). In the settings, you'll see a listing of all of the activities that the certificate can be tied to (see attached), which allows you to pick a particular grade that has to be achieved before it's unlocked. It's that easy.
I can't help you on the activity locking module as I've never used it.
Brian
Custom certificate (plugin)
Need Help with Certificate Module
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