As above..
M8 then Edinburgh City bypass heading onto the A1... should then be the third exit (Bankton Junction-posted-Prestonpans, Tranent, Port seton) - B6371
Try Here Bankton Junction
Follow B6371 all the way down to the T-junction , turn right along Port Seton main street (Gosford Rd) and the road to the harbour is the left turn before the shops start..