what extension are you trying to activate when you get that message?
Design Extender? Data Bridge?
The Active Domains ust shows a listing of the domains CMS has been activated on, there is no limit.
The extension allows you to install on 2 computers, that is the activvations used count. you can add CMS to unlimited sites.
try going to the all my downloads page and clearing the activations.
If you still have a problem I'll need more details:
What is your OS
What Version of Dreamweaver?
What extensions are installed?
What extension are you activating?
perhaps include a screen shot of the problem.