If you've seen the "you are on day X of your 15 day trial, here is the answer regarding the plugin's you see (those in the Units with checks in the boxes):
Thank you for contacting Sony Creative Software. There is no serial number for BorisFX that you will need. The message you are seeing asking for a serial number is if you are trying out any of the other effects from Boris not included with your Movie Studio 13 Suite Software. We apologize for the confusion with the message. You will still have your Boris FX Continuum Units even after this trial message expires.