Once you have ticked that the learner has completed the workshop, click in to the activity within the course which in this case is 'test submission':
...
In the next screen, click 'view all submissions':
You will then be taken to the below page where you can click the 'grade' button next to each user that has attended the workshop:
Here you can mark the task as either complete or not complete from the drop down dependent on whether the user has submitted the test, there is also a file upload option if Rob decides he would like to upload the users tests to the course as evidence of completion:
Once the 'complete' mark has been given, the course will show as complete in the users profile:
