Which FreeBSD port(s) are you referring to? I am unable to find a port entitled "claws-mail-extra-plugins" (or similar), so you will need to specify which port(s) you are looking for.
We are already including a number of additional plugins within the current Claws-Mail PBI[1], so you might want to make sure that it is not already being included. If so, the plugin(s) might not be getting integrated properly within the PBI.
[1]
http://trac.pcbsd.org/browser/pbi/mo...-mail/pbi.conf
(They are all listed after MKPORTAFTER=)