How do I place redstone dust on a dispenser? I am making a combat world and I want to make a TNT cannon. I tried tapping on the top of the dispenser, but it didn't work.
Hold Shift (or if you changed key bindings, the sneak key) while placing to be able to place redstone on blocks that you can interact with (furnace, crafting table, dispenser etc.).
On Pocket Edition:
Press the sneak button, it should look like this (it should be a dark circle with a light circle in it):
and not like this (it shouldn't be a dark circle only):
On console editions:
Press the right analog stick (until it clicks, down on D Pad on PS Vita) to toggle the Sneak mode.