When you visit https://www.iac.org/contests you'll see a blue 'Register' button next to every upcoming event. Just click it, fill out the required information, and submit the form.
To see who else has registered for an event, go to https://www.iac.org/contests, click on a contest name, then click the blue 'View Pre-Registrations' button. Contest organizers will see complete details, while everyone else will see basic info (name, category, etc.) Contest organizers will also have the option to download registration info in spreadsheet 'CSV' format.
Sorry registrars, we don't have an export-to-JaSPer feature just yet; that's next on the list.
As with any new software there may be some bugs or non-intuitive features. Drop me a line at webmaster [at] iac.org if you have any problems or questions.