As MEL is your entry into Australia you go through immigration, customs & quarantine.
Collect you baggage and go upstairs and to the left to the JQ check in area. A 3 minute walk.

United and Jestar are not partner airlines. Jetstar is a low cost carrier owned by Qantas.
UA is just being a travel agent in selling you a ticket to Adelaide.
UA Mileage Plus Gold member means absolutely nothing to JQ (or QF).
If UA have not bought you a fare with a bundle (baggage) you are out of luck.
Look at the www.jetstar.com web site and make a dummy booking.
Even if JQ accepts your checked baggage, you will be subject to the JQ carry on limits. These are a lot less than for USA based airlines

http://www.jetstar.com/au/en/plannin...hecked-baggage
http://www.jetstar.com/au/en/plannin...rry-on-baggage