The "instruments" are conch shells, turtles, and coconuts. I'm sure you found plenty! Just walk up to one, hit the Action key (down arrow or A button) and you'll pick it up. Then you can use A/down arrow to play random notes on it or hang it up on something from the environment (Windy Trees, Drippy Branches, or Bubbling Brooks) to make it play on loop (it will make a different type of sound depending on what you put it on) or hit Y to go into Music Mode. In music mode, all the keys on your controller face or WASD and up down left right arrows will control the music, and TAB or SELECT will escape.
I hope you try it again with that knowledge, and now you'll be able to enjoy it a million times more! I will be adding a tutorial mode after the jam to make sure everyone can fully enjoy Merlin's Musickal Machine!