This means that your STLport is not included properly in your include search path. I will remove dependency on <cassert> in the next release (so that STLport is not required for using the library, as opposed to building it).
For now: installing STLport is easy, take a look at the following link:
http://frog.flipcode.com/PocketFrog/install.html
If you have any other question, let me know!