The hydramatic uses Dexron automatic transmission fluid. Today, it's usually sold as Dexron/Mercon. To check the fluid run the car so it is at normal operating temperature. The filler/dipstick is located under a small square plate that is on the passenger side of the transmission hump. You'll need to pull back the floor mat for access.
Check the fluid with the engine running and the trans in neutral and the car on level ground. Once the access cover is removed from the hump you'll see the dipstick, it's marked with low and full lines. Add fluid as needed.
