I too have encountered a problem with Popcorn 3.
I upgraded to leopard a month ago. I noticed that Popcorn 2 wouldn't work in Leopard and that Popcorn 3 would. I purchased an upgrade and then downloaded the version which they stated would work. A few weeks later, when I would launch popcorn, it gave me a notice that the time frame was running out and that i should consult roxio for the latest update and gave me the options to continue or purchase the burner (which i already have). I contacted them about the situation and this is what they said:
"Drag your Popcorn folder to your trash then go to www.roxio.com then go to Support -> Software Updates, select your product and install the latest update."
I done that and now, what remains won't launch. Is their any idea as to what I can do?