The shack is located in the southern half of the McCarran airfield, connected to the Camp McCarran concourse through the aircraft boarding ramp. It can also be accessed from the tarmac, by passing through the Camp McCarran terminal building onto the tarmac in the back, then rounding the concourse building. The shack is impossible to miss.
It's a two-story shack filled with supply crates on the lower floor and Contreras' personal space up top, which includes a bed and a terminal. Sergeant Daniel Contreras is on the ground floor most of the time, dealing with logistics and his little side-business. There's no need to go back through the concourse if the player wants to move away from McCarran. Simply exiting through the rear door to the tarmac will allow for fast-traveling away.
Sunset Sarsaparilla star bottle cap in the closest wooden crate to Sergeant Contreras on the bottom shelf of the metal rack near him. Note that it is marked as owned and will therefore cost karma to pick up; this can be avoided by bartering with Contreras, which causes him to pick it up and offer it for sale.