Hi Cristina,
I'm so happy to see that you are excited about your upcoming trip - September will be here before you know it!
To answer your first question, you are able to purchase tickets to Mickey's Not-So-Scary Halloween Party online from the Walt Disney World website
HERE
As far as your online dining reservation is concerned, it will only appear in the "My Reservation" section if you were logged into the Walt Disney World website at the time that the reservation was made.
Either way, you should have received an e-mail confirmation when the reservation was made which will contain your Dining Confirmation Number. Once you have that number to hand, you should be able to retrieve your reservation on the Walt Disney World website by clicking on "Retrieve Dining Reservations" on the "My Reservations" page and entering the Dining Confirmation Number and your phone number.
So long as you have a record of the Dining Confirmation Number there is no need to worry. Simply arrive at the restaurant in question at the appropriate time with your confirmation number handy.
Wishing you a magical trip!