At ‘The Plain’ roundabout turn onto the Iffley Road. Follow the road down until you reach Jackdaw Lane, which is the first turning on your right (after about 300m). You will see the entrance to the sports centre about halfway down the lane on your right.
There is also a pedestrian entrance to the Iffley Road Sports Centre which you will come to before Jackdaw Lane. (You will come to this entreance on the right hand side of the Iffley Road after you pass Stockmore Road on the left).