This product is manufactured by Systweak. McAfee only sells it as an agent for them. Support is through them: http://www.systweak.com/VistaOptimizer/
However, as you paid McAfee and can't even get it to download you may wish to call Customer Service first:
To speak with a Customer Service representative, call 866-622-3911. You may be asked for your registered email address and order number. Telephone charges may apply.
Average contact length: 10 minutes
Estimated wait time: 5-10 minutes
Hours of Operation: Available daily, 6am-6pm Pacific
One thing to try although I can't guarantee anything as it isn't a McAfee product would be to make sure that Internet Explorer is your default browser and that it is set to its default settings.
If you have IE7 or 8 go to IE Tools/Internet Options/Advanced and click Reset, Apply and OK, also OK any prompts.
Close and reopen the browser and go to Tools/Manage Add - ons and re-enable the add-ons.
Try your download again.