opening times airport

Im arriving in narrita airport at 7pm on a saturday. Will the travel centre be oopen for me to get my japan rail pass (using the voucher i already have)
if not, im styaing in Mito- where is the nearest place to exchange voucher for pass?

Yes, the ticket counter, where you can exchange your voucher, is open from 6:30.

http://www.japanrailpass.net/eng/en009.html

Shows the closing times too.

Thanks but i've seen that info- a note at the bottom says ''Indicated Service hours are for weekdays. Hours vary among offices on weekends and holidays.''....im arriving on saturday 6pm

Ok, try this one then - it splits weekends and weekdays... Time don't change from the above link though as far as Narita is concerned.
http://www.jreast.co.jp/e/eastpass/obtain.html