On mine (1995 XT600E) it is on the left side of the carbs, next to the choke button, just above the float chambers. Look for a brass knurled knob pointing down and slightly forwards, with a spring around it to prevent it shaking loose. Yours may not be the same, but will be a similar arrangement.
Make sure the engine is warm before adjusting. Clockwise to increase idle speed, anti-clockwise to reduce. My manual gives the idle speed as 1200 - 1400 rpm, but without a rev counter I just guess it. Once that is done, throttle cable free play shold measure 3-5 mm at the throttle grip. As I said above, no slack is worse than too much slack.