Some users have reported seeing a message on Flixster telling them to activate their UltraViolet account, when they have already done so. If you are sure that your account is active, it may be that you have more than one UltraViolet account, and that the one you activated is not the one linked to Flixster. To activate the correct account, log in on www.uvvu.com.
If you do not know your login credentials, you will need to contact UltraViolet customer support. UltraViolet support can be reached here. Click on "Need help?" under the purple Sign In button at the upper right, then on "Help & FAQs" at the upper right on the next page. Scroll down to "Other" under "Contact Us" in the left sidebar on the following page, and explain that you have multiple UltraViolet accounts and want to make sure all of them are active.