The way controllers work in SP3 (the version of SketchyPhysics you're using) has changed since the last version, SP2 (which was used to write the tutorial you used). To give more flexibility, the controller field now accepts formulas, (i.e. math), combined with old-fashioned sliders.
To get the result you were expecting, use this formula in the controller field:
slider("name you want")It must include the quotes ("), but the name can be anything.
As a little example of what you can do with formulas, try making a motor joint, and using this as the controller (note that motors cannot normally reverse):
0.5-slider("Motor")Sorry, there's no official manual, but you can always ask here 😄