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. Most chapter pages have such an events sidebar.
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.