It works much like Skype or Google Chat. You don't need to install anything, it's all on the browser. On the day of the fair, make sure you have a bit of spare time, a laptop and a good internet connection. Just login, choose an employer and introduce yourself.
What can I do before the fair starts?
The employers will be offline when the fair isn't live but you can still let the company know that you are interested by submitting your contact info and scheduling a meeting a head of time.
How do I prepare for a meeting?
Meetings with employers are just introductions. They are quick and casual so don't stress over it. Start by asking some questions and find out exactly what the employer is looking for, if you have any relevant work experience, mention it as well.