Open the extension window, and scroll down a bit on the token list
Click the "Manage Crypto" button that appears
Click on the little + next to the "Wax" (make sure to pick the one not on Ethereum; which is the Wax network itself if not elsewise specified)
Once that's done you can continue to "Create an account" (this is also accessible from the enabled crypto list on the dashboard of the extension once the Wax crypto has been added clicking the +)
Choose an account name with 12 characters of length, limited to using letters a-z and numbers 1-5 (and all lowercase), and proceed clicking "Next"
Next step you need to confirm creating the Wax account (where a Wax/BCB/OXK collab is currently covering the costs, making it free
Read about the collab event here, and participate to take share of a 1M BRWL giveaway (valued at $5.5k USD at time of writing):