Turnitin works directly through D2L. When you log in to D2L with your StarID, you can submit a file on your course's "Assignments" page. Once you submit a document into the dropbox, Turnitin automatically checks your paper for improperly cited sources and potential plagiarism.
If your professor has enabled Turnitin icons for your class, you will be able to see the results of Turnitin's Similarity Report, which is what it uses to help your professor identify potential problems with citations in your paper.