(I didn't realize there was a fall run!)
As for modular arithmetic did you check out the free book by Victor Shoup (it's mentioned on the course site) ?
http://shoup.net/ntb/
As for professor interaction, it is a byproduct of the MOOC formula : don't expect too much from the professor, he is already teaching to thousands of students ! You'll have to manage your way through the discussion forums (who unfortunately have a lot of noise). During all the courses I took, I always managed to find answers like that.
Thanks for the link, for some reason I thought that book was on a different topic. I've found a series of lectures from Harvard Extension using the Artin Algebra book. I'm going to use that (and the new edition of the book) to pick up on the concepts I'm missing (over the course of the next few months)
The previous course I took with CourseRA had a decent amount of Prof and TA interaction, and the forums were well tended. But, I guess that could be down to the fact that the course was being used by the professor to instruct his actual University students at the same time.
As for professor interaction, it is a byproduct of the MOOC formula : don't expect too much from the professor, he is already teaching to thousands of students ! You'll have to manage your way through the discussion forums (who unfortunately have a lot of noise). During all the courses I took, I always managed to find answers like that.