Arobar Manor is on the right side of the Manor District next to the Redoran Council Hall. The helm is in the Manor Bedrooms, (from the entrance through the Guard Quarters, into the Private Quarters, and onto the Manor Bedrooms, Left Bedroom). The helm is found on top of a closet next to Miner Arobar, so you'll need to sneak in order to get it. If for whatever reason you have trouble stealing the helm because you get caught, it might help if you jump from the bed on top of the closet. This will ensure you are as hidden as possible. To take further advantage of this method, walk near the nearby NPCs and get them to speak to you, then when they're looking at you, move swiftly and position yourself where you would like them to be facing. Note: this only works for a limited time, after which the NPC will change the direction they are looking in.
Return to Aengoth with the helm for a reward of 1,000 gold.
Glitch exploit: Snatching the Master Helm can be pretty tough with Miner sitting right there if you do not have Chameleon or Telekinesis. If you face the closet, and go around the left side of it, you will be able to jump outside of the house. Simply stand in the corner that the closet creates on its left side and start jumping until you get behind the closet. No one will see you there, and you can get into sneak and grab the helm with ease.