Sent in a support ticket for this as well.

After installing the application, it asks you to activate it, which I do by providing my email / pass - then it tells me it was activated successfully, however the initial login box is still there. I close it and restart the application and it pops up again. I try to activate it by clicking the "provide serial number" and a separate login box comes up, asks for my email / pass / serial number which is provided in the downloads part of the store, and it says the key is invalid. I have never had this issue on previous beta software that I have tried or activated.

Regardless, I went ahead and enabled the evaluation version since I couldn't seem to activate it. The second issue I noticed right away after configuring it to show windowed mode in applications activated from desktop or windows 8 panel, is that when the application is active, it shows the icon associated with that application, however when you go to pin the application, the icon reverts to the (icon looks terrible btw) icon for Modernmix. Then I click the pinned icon and nothing happens.

So at least from my experience, it seems there is an activation bug, as well as a bug that seems to have broken the ability to pin the applications to the taskbar.