There are 4 options on the STC-1000
F1 - Set temp
F2 - Differential
F3 - Compressor Delay
F4 - Calibration
Press and hold that S-button until you see F1 on the display. You then release the S-button and can use the up or down arrows to move from F1 through F4.
If you have F1 showing, HOLD the S button and while holding it (for me this takes two hands) press the up or down arrows to pick the temperature you want it set at. Once you have the set temp you want, release the S-button, and press the power button once to save it. If you forget to press the power button when you're done, the STC-1000 won't save your change.
In normal operations, the STC-1000 display will always show the actual temperature reading from the probe. The only way to know your set temp is to get into the settings like this.
[Edit 1] I just remembered you can make the STC-1000 show your set temp by just pressing the up or down button (I forget which) while it's in normal operation mode. You can't change the settings this way, but it will display the set temp for you.
F2 is the temperature differential, as I mentioned. Same deal. Press and hold the S button until F1 shows up. Release the S button, and press the up or down arrow to get to F2. Once you're on F2, HOLD the S-button again and press the up and down arrows to pick a differential. 0.5 or 1C is usually fine, but fine tune it to your system. If you have it set at 1.0, and the temperature of the liquid gets 1C over your set temp, the cooling plug will kick in. The reverse is true for heating.
F3 is the timer delay for engaging the cooling unit, and is to preserve the compressor of your chest freezer, fridge, AC unit, etc...
F4 is temperature calibration. Always test your temp probe with a known temperature, boiling water, freezing water, or something in between using another temp probe you trust to compare. If it's off, you can make adjustments.
Happy brewing!
[Edit 2] When you test the STC-1000 don't freak out if your cooling outlet doesn't power up right away. It will wait for the differential period before kicking on. If you're differential is set for 7 minutes, wait 7 minutes before you go ripping wires out! Like I did for 2 hours before I figured out why my cooling outlet wasn't working.