To run ...
Download the file jwebmail-1.0.1rc1.zip
from
SourceForge, and read the very brief
binary install section
of the User's Guide.
To modify ...
Either export or check out the trunk branch from
Subversion
at Sourceforge, or
download the file jwebmail-1.0.1rc1-src.zip
from
SourceForge.
You can learn what you need to know in the
User's Guide.