A properly installed and sealed A/C unit will not leak. No matter how hard it's raining.
With that said. Remove the inside cover to the A/C unit. Look up and you will see the head of some bolts. These bolts will snug up the gasket between the unit and the roof.
See if you can see any signs of water. Snug the bolts by perhaps a half turn.
Use a garden hose to flood the area with water. See if any leaks inside. If so, snug the bolts another half turn and use the hose again.
If it's the gasket, this should fix the leak as long as the gasket isn't damaged.