If you go to the assignments page, the ones that start with "ML:" are the mystery language assignments. They're online and you can play with them right there!

For the 2017 course [1] we re-implemented the mystery languages in Racket, so you'll need to install DrRacket to use them. But they're a bit improved and more robust.

[1] http://cs.brown.edu/courses/cs173/2017/

As @justinpombrio says, I would totally go with the 2017 version instead. Not only is it much better, we are actively supporting and developing this version, so bug reports are actually useful to us and future users.