Arms: Sharpen Blade - When activated, your next Mortal Strike will deal 15% increased damage and reduce healing taken by 50% for 6 sec. Fury: Battle Trance - You go into a trance causing you to regenerate 3% of your health and generate 5 Rage every 3 sec for 18 sec after using Raging Blow twice in a row on a target. Using Raging Blow on a new target will cancel this effect. Protection: Thunderstruck - Thunder Clap roots all targets for 1 sec. You can find a list of all Essences available in Rise of Azshara here.
Like with addons in the current game, you just need to drop any of these addons into the addons folder — find it under /Interface/AddOns/ in the Classic beta folder, which is in your World of Warcraft folder by default. Then once you log on to the game, you’ll want to make sure that the addons are enabled via the menu option on the character screen.