This is assuming you have an automatic tranmission, not a manual transmission.
If you find the transmission drain plug, the overfill plug is about 6" from the drain plug. It should look just like the drain plug. Be sure that it is the correct one.
See image in link.
When adding or changing fluid, use only DEXRON III.
CAUTION: DO NOT OVERFILL. Overfilling will cause foaming, loss of fluid, abnormal shifting and possible damage to the transmission.
Park the vehicle on level ground and apply the parking brake firmly.
Check fluid level with engine running at idle.
NOTE: Be sure that transmission fluid temperature is below 86°F.
Move the selector lever through all gear ranges.
Move the selector lever to "Park".
Let engine idle for 3 minutes and open the overfill screw.
Add released transmission fluid until it flows out over the overfill screw opening.
Let engine idle until a fluid temperature between 90°F and 135°F is reached, then close the overfill screw.
Torque: 28 ft. lbs
NOTE: Check transmission fluid temperature with scan tool.
Minimum fluid level 135°F
Maximum fluid level 90°F
CAUTION: Do not open overfill screw with engine stopped.
CAUTION: DO NOT CHECK FLUID LEVEL UNDER THESE CONDITIONS:
Immediately after driving at sustained highway speeds.
If vehicle has been in heavy city traffic during hot weather.
If vehicle has been towing a trailer.
If the vehicle has been operated under these conditions, shut the engine off and allow the vehicle to cool for 30 minutes.