I am not aware of a Tool called "Dissolve" in Photoshop. Are you thinking of another tool or are you thinking of the Dissolve blending mode, which can be used with a brush or a layer? Or something else?
Please clarify and show a screen shot of one of these, depending on what you mean:
Tools panel with the tool selected
options bar with the Brush tool selected
Layers panel showing all layers and select the one with Dissolve
Or whatever else you might be looking at if it's none of these