to be honest pal, id go to a boozer near the ground, have a pint and ask the bar staff to ring you a taxi, take the taxi to Mills Hill Train Station, cost about a £5, then from Mills Hill its a 10-15 minute train journey to Manchester, and as stated earlier, the train station is directly under the MEN, so walk up the steps to the Arena.
The bus isn't a bad shout either, but if you don't know Manc your not gonna know where to get off, where the Arena is etc. But then again, im a fuss pot and wouldnt want to get lost lol