Currently Plumrocket uses Serial Key for activating extensions.
Once you have purchased and set up your Plumrocket Magento Extension - normally it will not require setting up the serial key, since it will be automatically pasted into the relevant field under the configuration section of the Extension. If so - you will see a green tick next to the serial key. However, if not - you will need to find your serial key and paste it manually. More information on serial key setup can be found in this Wiki Article.