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.