Are you taking an in-person class? If so, you can ask at the office for another pen (if they have any extras lying around - though they may not).
If you're taking an online class, call 800.576.GMAT or email
studentservices@manhattangmat.com, tell them your name so they can look you up, and ask them either to send you another pen or to tell you where to get another similar pen if you don't want to wait for one to be sent to you.