Is rinse aid leaking from the cap or from the dispenser?
If you can not tell where the rinse aid is leaking, remove the cap, fill the dispenser, then replace the cap. Close the dishwasher door, wait a few moments, then open the door and inspect where the rinse aid is leaking. A way to tell if your rinse aid dispenser is leaking, is by the amount of rinse aid you are using in a week. If you are having to re-fill your rinse aid dispenser, on average, every week, your dispenser might be leaking.