missing preset slider bar in LrC

Not every preset supports the slider. First of all, you have to turn it on when you create the preset, like @Sean McCormack said. But even then the preset may not show the slider. That's if the preset does something that cannot be done in parts. Let's say that the preset applies a raw profile. You can't apply 50% of a raw profile, it's all or nothing. You either apply this profile, or you apply another. So in that case I would expect the preset not to support the slider.

The Support Amount Slider needs to be turned on when creating the slider for new presets. I'm honestly not sure what happens with older presets though.