I've had similar trouble getting pins 24 to 33 to work with pduino on the arduino mega. I have found, however, that pins 34 to 53 can be activated. You just need to add in some more [inlet] and [digital 34 $1( etc, in the same window where you edited pin one to be pin 24. Doing so automatically creates new inlets on the [pd generate digital out messages] object.
I've uploaded a rough example that includes pins 34 and 53. I hope that helps.

http://www.pdpatchrepo.info/hurleur/arduinomega-test-experimental.pd