To access student submissions from within MedLearn, you must first add the Turnitin **submission class** for the assessment(s) that you are marking to your Turnitin profile.
This can be done by logging in to Canvas and opening the page which contains the Turnitin submission portal for that particular assessment.
This only needs to be done the first time that you are accessing any assessment in that module, and will need to be repeated for each module you're marking.