[plt-scheme] arrow.ss teachpack

I have been using the arrow.ss teachpack to do the Lunar Lander exercise in
Section 21.4 of HTDP. I evaluated exactly what was written in the book to
control the lander up and down and it was giving me all kinds of error
messages, which seemed to contradict the information in the Help Desk of
DrScheme on arrow.ss. Can anyone give me some insight on this? I am using
the language Intermediate Student with lambda.

-Connor