ha!
i was having the same problem as well it took me all day to figure out what i did the first time i installed the plugin!
anyway, when you add the "body paint" tab to your may user interface (top bar where file, edit and modify, etc..)
"clic" > body paint >send to body paint> options
make sure your boy paint path is : C:/Program Files/MAXON/BodyPaint 3D R2.5/BodyPaint 3D.exe
first make sure you download this plug-in: http://www.maxon.net/pages/download/bp3d\_e.html
ad one file to the : bin / plug-ins file.
and the other files to the : scripts / start up folder
if all these are done it should work, and you should be able to work with both programs seam-less-ly!
ha, i actually know somthing!