Make sure that you're inside the Skyblock gamemode first. Then, in the ninth slot of your inventory there should be an item that looks like a piece of paper with a tree in the corner. That's the Skyblock menu.
If it is not there, here's what you need to do: go onto your Skyblock island and interact with the
Skyblock shop. Choose the "Player Settings" option (7th option). Then toggle on the "Hotbar Item" option. Make sure the last slot in your inventory is empty before doing this though.
Hopefully this works