Just like loading ammo onto the slingshot, you have to have both the bait and pole in your inventory. Open your inventory, it does not work from the toolbar. Touch, hold, and drag your stack of bait onto your pole. The pole should light up, and the bait should go on the pole when you release.
I can't add more bait- whatever I try to add will just replace what is on the pole. So I take one piece of bait, put it on the pole, then combine stacks in my inventory. Then I replace the single bait with the combined stack. Same with the slingshot & ammo. Also I can't figure out how to take bait or ammo off, other than replacing it.