Some undergraduate UMUC courses may require that students enroll in an instructor's Turnitin class and submit work there for checking. IF your instructor wants you to do that then he or she has to create a Turnitin class, create assignments, and has to give you instructions on how to ENROLL in their Turnitin class. See this portion of UMUC's Turnitin FAQ for the steps:
In the graduate school instructors do not require students to use Turnitin. Instead they may upload a student's work to their own Turnitin class on a case by case basis when the need arises.