by Davo Smith.
I've already answered this question in my last response, but, as with all the core activities that use grades, you need to call the myplugin_update_grade function in you add_myplugin_instance function and in your update_myplugin_instance function (in order to create the grade item for your activity in the grade book), you then need to call it again at the point where a user is awarded a grade (I can't tell you where to put this, as it depends on how your plugin awards a grade to a user).
The first two should pass 'null' as the 'grades' parameter. The last one should pass an array of grades as the 'grades' parameter. Again, look at the examples in core to see how to do this.