Where do you add transmission fluid in a 91 Mazda Miata?

On manual transmission Miatas, there are two bolts on the driver's side of the transmission.

The upper fill hole bolt's head is shaped like a square, and you'll need a special, 8-pt socket to remove it. It is strongly advised that you remove this first. You want to ensure that you can actually refill the transmission before you drain it! It is also very common that this plug is extremely stubborn. Make sure you have a good breaker bar and loosen it with rust remover well in advance.

The lower one is a standard bolt that is to drain the fluid.