During My Arm's Big Adventure you will first meet My Arm when he is Burntmeat's assistant. Burntmeat asks you to teach My Arm how to farm Goutweed for use in his cooking. When this is eventually achieved, My Arm can always be found growing a patch of Hardy goutweed on the roof of the Troll Stronghold.
During King of the Dwarves you will have to ask My Arm how to get a troll named Pretty Flower to talk to you and he will suggest giving her a Pretty flower.
During The Mighty Fall My Arm can be found in the Dorgeshuun Camp in Yu'biusk as he's come to cheer you on during the Kyzaj Tournament.