Hi Guys, I had similar problems to all of you. I bought a boxed version of Popcorn 3 today from the Apple Store. Installed it, but it would not open. I managed to register it and that was all it would do, apart from open the label designer.
To get it to work.... go to Roxio, download the latest update which is Popcorn 3.0.2.
Once you have downloaded it open the update folder, copy the popcorn file into the popcorn folder which you have ALREADY installed. It will ask you to replace the file and you will say yes.
That should get it working, it did work for me.
Sorry if this all sounds a bit basic - I am rather new to a Mac.
By the way I am furious that they must be aware of this issue but are not telling people as a matter of routine to install the update. I just could not understand what I was doing wrong!