I just tried the link you provided and it took me to Arcade and I started a game and the questions loaded fine.
I wonder if it has something to do with your login credentials, some cookie in your browser. I would maybe suggest logging out of your student center, clearing your browser history / cache / logins, restarting your computer, and trying again.
Let me know if problems persist, or email
techsupport@manhattanprep.com