Hi Diana,
I don't know if you've noticed - like me you probably had an older lightning.xpi file from Postbox 3 - but Postbox Inc are no longer supporting Calendaring in PB5 - according to them lightning is not in active development any more, so they have removed it from their list of add-ons, and the FAQ now reads as:
Does Postbox include a calendar?- Not at this time, as we're solely focused on providing you with the very best email experience possible.
So, I reckon that getting the Google Calendar provider supported in PB5 will become more difficult because of this.
I've tried to use several (older) versions of lightning and modifying the install.rdf file in the .xpi file as outlined on the PB Add-ons page, but to no avail. I didn't get as far as doing the same for this provider.
Darren.