To register for Unicon, you need to sign up for an AON membership. This allows you to register for Unicon - and many of the other conventions around Melbourne - online, using your membership to register for each convention as it comes up with the same username and details.
If you have already got an AON membership, you should leap straight in and login to the system.
If you're new to AON, then you should probably peruse the information about AON before moving on to signing on up.
You can't pay for Unicon through the online registration system, but that unfortunately doesn't mean its free :-).
The charge for each session covers the cost of room hire, the administration of the con and prizes - all the efforts of the writers and conorgs are supplied for free by the dedicated volunteers.