I think you might be out of luck. Here's why:
The National Rail website states that there's no car park at Highbury & Islington station. The postcode is given as N5 1RA.
Putting that postcode into Multimap (which can provide details of car parks, cash points, filling stations, etc) shows no car parks in the area.
Putting the postcode into the NCP website shows their nearest car park as Kings Cross station.
The stadium website provides travel directions by public transport but not by car:
http://www.arsenal.com/article.asp?thisNav=tic kets&article=344177
The Islington Council website makes it clear that there's no street parking in the area on match days but offers no alternatives:
http://www.islington.gov.uk/Transport/RoadsAnd Parking/Parking/ControlledParkingZones/MatchDa yParking/default.asp
I think that you'll need to park well outside the area and use public transport to the stadium. (That was always the plan when Arsenal sought planning permission. Nobody is expected to drive there).
Chris