Take the skybus to the southern cross station, than its a pretty straightforward 20min walk to the hostel !
The Skybus ($19 one way or $36 return) takes roughly 20 to 30 mins to get from Tullamarine Airport to United backpackers.
If the Skybus is doing hotel transfers, asked to be dropped off at Doubletree Hilton, (270 Flinders St) which is a minute walk from United backpackers.
It's about 30-40 minute drive from Tullamarine airport. You can get the SkyBus for $19, then it's a short walk to the hostel. Taxi will cost round $70 I think.
If you are flying into Avalon, it's a lot further out- not sure about transport options from Avalon.
Get an answer from United Backpackers Melbourne staff and past guests.