Hi William,
The screens for the default Contacts Portal are in the format ContactsPortal_<Name> for QBD and ContactsPortalQBO_<Name> for QBO. They can be found in Method by navigating to Customize > Screens.
If you'd like to add a new screen for your calendar, you can do so from here. Otherwise, you can copy & edit one of the pre-existing screens and place the calendar on there. Please keep in mind that if you create a new screen, you'll have to add it to the Contacts Portal tab by navigating to Customize > Tabs before it will be visible.
Once you're in the screen designer, click Add Objects in the toolbox on the left and drag a "Calendar<#>" object over to the right side of the screen.
From here, click Edit in the top-right of your Calendar object and step through the Calendar Wizard to set it up.
Once you're done configuring, Publish, Save & Close and view your screen to test it out.
Hope this helps.
- Justin