Sounds like you're trying to use the original calendar archive written by pixelpunk, and that one does not work with version 1.5.
Download the one from this link
, and you should have better success.
I've gotten it working on my version 1.5 site, which you can see here
. (I haven't gotten this finished yet, as version 1.6 came out, and I want to use the latest version on this blog.)
I did have to do a few modifications to it in order to get it working and to display the way I currently I have it.
Let me know if you still have problems, and I'll see what I can do to help.