You cant. But you can easily create an appointment on :15 :30 :45 markers.
1. Start typing the appointment in an adjacent hour
2. Hold ALT (moon key) and spin the scrollwheel to change between :00 :15 :30 :45
No need to open the appointment to edit it. It can all be directly done in the standard calendar day view.
More information in BlackBerry Calendar Tricks.
Problem solved! (hopefully)