I have sent & received several appointments via email with my daughter - can't remember exactly but I'm purty sure I just opened her email message that included the meeting request, and pressed the BB menu key, and "accept" is the highlighted option, click it, and the meeting appears in the calendar.
at the bottom of the invite, there are a few options.
the symbol with the green check mark between the 2 people, accepts the invites.
the symbol with the yellow question mark between the 2 people, sends a response of tentative.
the symbol with the red X between the 2 people, declines the meeting.