Okay here's what I've been able to figure out from the information in your first message and some leaps of thinking that I've made. According to
SearsPartsDirect.com this is the manual for your mower
Operator's Manual and it's a:
ZTS 7500
Zero-Turn Rear Engine Rider with Electric Start
Model No. 107.28791 (26 HP Kohler Engine with 50" Mower)
According to page 29 of that manual:
Transmission Type: Hydro-Gear EZT ZC-AHBB-2A7B-1DPX
According to
Hydro-Gear.com this
might be the manual for your transmission
ZT-2100 / ZT-2200 (EZT) Integrated Zero-Turn Transaxle Service and Repair Manual.
If this is the correct transmission manual the diagram on page 4 shows where the fill port is and the fluid change procedure is on pages 11-12.
Hopefully this can be of some help to you.