Oh dear! I just went into the arcade on my chrome browser and it worked just fine. I'll pass the issue along to our tech support team and see what they have to say. If you can, forward a screenshot of the error message to me at: ldamone@manhattanprep.com!
What course are you enrolled in? There isn't anything in the Arcade that's critical - all Arcade assignments are "Extras" in the syllabus so don't stress if you can't complete them. Focus instead on the drills in the 5 lb book - those are like the arcade drills but include explanations!
Hope this helps!
Laura