Q: Trying to purchase a calendar from iphoto

but it's telling me I need to install something else.

What is the exact error message? And what is your iPhoto version?

Maybe, you need to upgrade to a newer version of iPhoto. You can only buy calendars, books , etc from the Print Products store, if you are using iPhoto '09 or '11, see this link:  Product Line Details for Books, Cards, and Calendars

iPhoto '07 is no longer supported for buying Print Products.