The Idle Air Control (IAC) valve on a 1994 Chevy S-10 is located on the throttle body. It's a small, usually cylindrical, component that's attached directly to the throttle body itself. You'll likely find it on the side of the throttle body, opposite the throttle cable. It might have a single electrical connector plugged into it.
Because there are variations depending on the exact engine (4.3L V6 or 2.8L V6 for example), the best way to find it is to look at a diagram of your specific engine in a repair manual or online. A quick image search for "[your engine] 1994 Chevy S-10 throttle body" will probably show you exactly where it is.
