In general you will use straight thru cables from the iPockets to the serial ports.
Use the attached PMS system emulator from Mitel to test the iPocket connections, if you can talk to the 3300 with no issues with it, then it's on their end, if it doesn't work, your iPockets or 3300 are not setup correctly.
Pretty much with PMS interfaces we verify they work at the iPocket, and go no further... from there the PMS vendor handles it.