To get your serial number you will have to go to your Download Center. This can be reached off www.webassist.com and going to "Your Account" and the My Downloads.
The serial number will be listed next to the product name. In Dreamweaver when you open up any of the toolkit's functionality, you should be prompted to enter your serial number. You can just copy paste the serial number from your Download Center into the first text box on that interface.