The exact location of the modulator varies slightly depending on the specific transmission case design, but it's generally located on the driver's side of the transmission, near the top or side. You'll likely see a small vacuum line connected to it; it's usually a fairly small-diameter line.
To be certain, you'll need to visually inspect your transmission. A shop manual specific to your 1994 Camaro Z28 would be a huge help in locating it precisely.
