Most of our plugins have a 'Register' option in the Effect Controls panel, regardless if they are registered or not.
This is primarily for trial users to register their product out of trial mode or to see how long they have left in their trial period (when applicable).
It is also useful for Registered users to see which license is in use, to help decipher if two machines are running the same license concurrently.
As long as you are not seeing on watermarks on your output, you can ignore the Register button.
If you do not see a 'Register' button and wish to pull the plugin out of trial mode, you can close whichever host you had installed the plugin into, and enter the serial into the installer you'd used to get setup with the product.