Post an event
Most chapter pages list regional events on their left sidebar, for example http://publiclab.org/wiki/gulf-coast. Look in the sidebar for the "post an event" button to add yours. If you have recently created a new region page, email email@example.com to request a "post an event" button.
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.