Post an event
Any page with an events listing (they are by region, like this: http://publiclab.org/wiki/gulf-coast) has a "post an event" button in the sidebar. This system isn't fully built out yet, but those events are then listed in that sidebar.
We also have a Google calendar, which is how you get listed on the http://publiclab.org/events page.
How to host an event
Want some advice on hosting an event -- how to reach people, what to provide, how long it should be, etc? Take a look at the Host a Balloon Mapping Workshop page for some great tips.