Cowfold Seagull
Fan of the 17 bus
If you're staying in Brighton, and it's likely you are, then I'm not sure you'd not want to head over to Lewes pre-match when there is such a wealth of decent places close to Brighton station.
Assuming that to be the case [MENTION=37925]CricketFieldEnd[/MENTION] - stay in the vicinity of Brighton station pre-match. Decent places includes the Battle of Trafalgar, Prince Albert, Sussex Yeoman and the Lord Nelson - all just a few minutes from the station, but there are loads more.
Get to the station by 2pm as there will be a queueing system for trains to Falmer, which is right next to the ground. You don't need to pay - it's included in your match ticket.
Post-match - stay behind for a bit and let the train queue back to Brighton recede. The bar in your end will be open, but so are the bars in the Albion ends, which you're welcome to use as well. After a couple of pints or so, you shouldn't have to wait too long for a train to take you back.
I honestly think that this is the only post that the OP needed to read. Sensible and to the point. Al of the other conflicting suggestions as to pubs and locales, are only serving to completely confuse him!