Basically, if you follow signs to the National Watersports Centre, you won't go far wrong.
From Nottingham
Go over Trent Bridge heading out of Nottingham. Turn left just after Trent Bridge on the A6011 towards Grantham. Follow the signs to the National Watersports Centre. You should turn left onto Regatta Way. Follow this road along towards the watersports centre. The road forks just after the campsite. Take the left fork towards NWC Reception. The Nottinghamians Ground is on the left hand side just before you arrive at the NWC gates. The car park is signed - the ground is shared with Nottingham Cricket Club.
From Grantham