Your access to Mastering Physics comes with a code attached to your desk copy of the textbook. You might need to register first. You might also have a class from a previous term that by default you log into, and you will have to change to the new class. Use the "Courses" button in the menu to the left.
Once you log in you need to set up your course. There is a "Manage this Course" button to the upper right that will allow you to set class properties, add or edit assignments, add class assistants, etc. This button is typically not shown when you first log in, but it will appear after you press "Assignments" in the menu to the left. There is also an equivalent "Choose" button that appears on choosing the "Courses" option in the menu to the left.
One thing you do not need to do is provide a class roster. Your students will register themselves, with the course ID that you have chosen and given them.
You should look into how gradebook assigns credit for the students answers and limits their number of attempts - perhaps the defaults are not what you want.
To get the grades from Mastering Physics in to the department gradebook, do the following in Mastering Physics:
Mastering Physics allows you in setting up assignments to choose from the questions, problems, etc at the end of each chapter, often with the numbers somewhat randomized. One issue with this is the precision with which the answers must be input. Often gradebook complains about the precision of answers. Sometimes this leads to problems for the students.
A more serious issue that you should consider is that a large number of the students, about 2/3 - 3/4 from my experience in spring 2009, manage to get 98-100% credit for their Mastering Physics performance. Based on the student grades on quizzes and exams, the overwhelming majority of those with excellent Masting Physics scores have a limited understanding of the subject matter, so their performance here does not reflect their understanding.