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, 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.
(They are all listed after MKPORTAFTER=)