You can't pay for a license. CC is by subscription model only so you have to join the Creative Cloud.
If you haven't used up your 30-day trial yet, you should be able to activate the software by clicking on license this and logging in with your Creative Cloud ID and password.
Adobe does still offer DWCS6 as a perpetual license product, but it's 4 versions removed from current and is no longer receiving updates.
It will still work, but it's starting to show its age.
You could purchase it here if you like: Creative Suite 6
If you have purchased Creative Cloud, and Dreamweaver is asking you for a serial number when you open it, use this document Creative Cloud applications ask for serial number for troubleshooting. If you still have problems, let us know.