BarnOwl plugins will typically be distributed as a .par file. To install a plugin, make a directory ~/.owl/modules, and place the .par file into that directory. Make sure to preserve its name -- this is significant to BarnOwl.
To load the module, from within BarnOwl, execute the :reload-module MODULE-NAME command, where MODULE-NAME is the name of the .par file, without the trailing .par.
If anything goes wrong, you may be able to get more information about the error using :show errors.