There are generally two reasons why this happens.  For one, ProTools gives this message any time our authorization process doesn't complete. Please be sure to click "I'm online" and enter your login info when the Activation prompt comes up during startup. If you click "Quit" instead, you'll get that "not a valid 64 bit plugin" message. Is that what you did?  

If not, we'll move on to the second possible reason... It's possible that the plugin somehow did not install properly, and the AAX file became corrupted.  If that's the case, you can just manually uninstall it and then reinstall.  Just go to this directory location:

C:\Program Files\Common Files\Avid\Audio\Plug-Ins

This will open a folder with all your AAX plugins, and you can throw away the plugin file.  Then try installing it again.