How do you find this sixbot for the atomic wrangler ? The quest marker shows me nothing it just keeps telling me to go in the freeside doors and then back out... lol

Go around old mormon fort you will eventually stumble upon the place it starts with a c I can't remember the name. I'm not sure what the deal with the marker is because its wrong maybe its a bug?

To get to the building with the sixbot you need to be on he side of the freeside door where the NCR was giving out food, keep following the road leading to that place and you should find it, its really badly placed, had to google it myself

Where do you get the key from to enter the building?

I assume you mean the ruined shop. You get access to that building when working for The King.

As for the six Bot it can be a hassle to find the structure hes in. From the Atomic Wrangler head further down the road (away from the criers) and turn right into the last building. Go through the door on the first floor. Take a left and follow the road all the way round past the rubble and it will be behind a ruined bus at the very end.

Look at it with the local map and you will see where to go and why the quest marker is where it is. This confused me at first as well.

Cerulean Robotics is the name of the building you're being sent to. The door is in a small back alley somewhere in Freeside. The marker isn't wrong, it's just pointing to an odd location.