You make a new one...
You make a new layer, use your splatter brushes, make a new layer, go to Apply Image, then right click the applied image and click clipping mask.
D:
So I use the brush on any layer, or do I make a new one?
After this I click the layer, and go to image and click apply image, and then right click the layer with the image or brush splatter? then I click clipping mask