Highlight the event so that it's in the information panel >> click on the up/down arrows next to 'alarm' >> click on 'message with sound' in top half of drop down menu >> click on 'minutes before' to select the time scale >> click on the number in front to set it.
Eg. for an hour and a half, select minutes and then 90.
EDIT: sorry, just noticed that it's in the iphone forum, not a problem setting the calendar.