To display times in am/pm format, go to Admin->General->Date and Time and set the parameters there (click on the help link on the bottom to see how the syntax works).
If users are going to input events and use time selectors to set the event times, those time selectors are only in 24-hour format in version 6.x. But an option to show these selectors in am/pm format has been added for the next version (7) that is expected to be released sometime in May.
Hope that helps,
I have just volunteered to create a website for a local senior citizens group. They would like their site to include a calendar... but there is no way these folks are going to comprehend times in 24 hour format.
Is there a calendar switch somewhere for AM/PM time display?