oh yeah, seamonkey is a whole suite.. you mean the default string opened seamonkey's composer- the design editor?
try this:
in kmeleon click on tools>mail and news>configure mail
in the buffer, first type the location for seamonkey, then add this paramaeter: -compose %1
example:
c:\program files\mozilla\seamonkey.exe -compose %1
also check this link for seamonkey's email parameters:
http://forums.mozillazine.org/viewtopic.php?f=40&t=489345