If you are trying to run the Quick Start tutorial from the Cactus web site, I believe it no longer works.
Here's a mention in the release notes of 1.8.1.
Release Notes - Cactus - Version 1.8.1
Bug
* [CACTUS-124] - Cannot run the Servlet Example
I get the same error message with version 1.7.x.
I just started researching this product, so I'll post an answer if I find one. Hopefully someone else will have an answer with the extra info I have provided.
EDIT:
Found a work around. Looks like it's an issue with package names. I re-compiled the two files using the default/no package and it worked.
EDIT 2:
I looked up CACTUS-124 in the bug database and it's not related. Also I believe only the class files that extend ServletTestCase need to be changed to the default/no package name.