Is the printer driver properly installed?
Open the "Setup Utility / Print Center" and then check if the printer driver for your P-touch has been registered.
Is the correct printer selected?
Select the "Printer Setup" from the "File" menu.
Check if your P-touch is selected under the "Choose Printer".
Is your P-touch directly connected to the Mac's USB Port?
You shouldn't use the USB ports on the Mac keyboard.
When connecting your P-touch to the Mac via a USB hub, a proper connection may not be possible due to the type of the hub used.