Hi Chris,
Although you don't specify which Add-on Data Pack you are using, I have attached the install insructions for the Retail MarketPlace Add-On. The steps should be the same for your Add-On.
One issue we encountered on Vista and Win 7 machines, is the one you mention in your post. The resolution is to disable the UAC (User Account Control) prior to installing.
Please let me know how this process goes for you.
Thank you.
Kristen