That might be where the problem is... eCart 5 cannot be found under my "archived products". All my other older products appear to be there, including eCart 4. I checked in "My Orders" and it looks like eCart 5 was purchased back on Aug 24th, 2010 - it's just not listed in my account anymore.
Could that be why I can't activate? Is there a reason why it's missing from my account?