There are two types of packages that can be loaded by plasTeX: 1) native LaTeX packages, and 2) packages written entirely in Python. plasTeX first looks for packages written in Python. Packages such as this are written specifically for plasTeX and will yield better parsing performance as well as better looking output. Python-based packages are valid Python packages as well. So to load them, you must add the directory where your Python packages are to your PYTHONPATH environment variable. For more information about Python-based packages, see the section 4.3.
If you have a true LaTeX package, plasTeX will try to locate it using the kpsewhich program just like LaTeX does.