If you haven't yet registered, you can go and talk to Lotto Bob to do so, but you'll have to pay a hefty fee of 500g.
Hefty to me as I am always running low on gold ._.
So the lottery will be held on the 15th of July and you may only partake in the event if you have registered by July 14th at 11:59pm server time.
Also, you can only win your prize on the 15th of July, any later and your registration will be void.
Good luck! :3