If you select a layer and press e e - that is, press the e key twice, it will show you all of the properties with expressions.
It sounds like you are starting an expression on a property that doesn't have one already. If there's no expression, you'll need to add it from scratch. Thta is, delete what's there and replace it with something like wiggle(2,9)
If there's already an expression on there, alt-clicking the stopwatch is not what you want to do. Just twirl it open to reveal it.
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