I'm trying to animate the levels of a threshold adjustment layer to go smoothly from 1-255 and back again. I have the timeline open but can't even seem to get it to change from one keyframe to another and every tutorial is too complicated to understand. Any help? Thanks
You cannot animate adjustment layer properties, only transform properties, opacity and a few other things. What you want to do is simply out of Photoshop's capabilities.
A better option would be to use something like After Effects, which does exactly what you want and would give you much more control.
A workaround in Photoshop...
You can't animate adjustment layer properties, but you can animate layer styles. So...
Create a threshold adjustment layer set to "255" (i.e. all black).
Place a white layer behind your image (this is where the white will come from).
Open the Layer Style dialog for your image and under "Blend if: Grey", you will see that adjusting the white slider will change how much white shows through your image. Since we can animate layer styles all we need to do is animate this slider from one extreme to the other (and back again)...
Simply place three keyframes on the "Style" property in the timeline—at the beginning, middle and end—then adjust the "Blend if" slider appropriately at each keyframe.
Note, I'm using a single image but you can just as easily place layered artwork in a group and adjust the style of the group, there is no need to flatten your artwork to a single layer.