The Ruined store is a destroyed building in west Freeside. It is located near Cerulean Robotics.
The only entrance is locked and requires a key to open. During the G.I. Blues quest there will be 2 NCR Troopers standing outside the entrance. In order to enter the building to talk to Elizabeth Kieran you must pass a Speech check of 60 (or 30) from one of the troopers, you can pickpocket a key to the store, or you can obtain the password from the Missionary after answering his three questions to convince him you are an New California Republic (NCR) citizen. Inside you will find Squatters receiving free food and drinks from Elizabeth Kieran.
Finding the store is not easy if you rely on the Pip-Boy. Approaching it from town guided by just the compass markers will make it look like you are there already. To get there from the Silver Rush: looking out from the Silver Rush doors, face North, cross the street and enter the building where Bill Ronte (the one drugged out on whiskey for the High Times quest) is sitting on the floor. Take the door located on the ground floor. On the other side, head North, and then turn left under the motorway bridge pillars. As you reach the burnt out car (next to the train tracks), turn left and follow the road heading South West, parallel to more motorway bridge pillars. The building is on the left hand side.
If you're having trouble, follow the train tracks towards the building with the barred up blue doors. The NCR building is opposite the South Eastern corner of this building, over that bit of rubble.
Behind the scenes
The squatters in the ruined shop were supposed to queue up for the food according to the script notes, but it seems there wasn't enough time to implement it. The undercover NCR troopers oddly have a dedicated idle marker for sitting on the curb on the opposite side of the road to where the ruined shop is. They never actually use this marker and it doesn't make much sense for them to - the only time they appear in the game is to guard the ruined shop once you know about their actitivies in town, not lounge around in the road. Interestingly the marker used for this is in the original FreesideWorld map, in an area that is inaccessible in the final game (that part of the level was split off into the FreesideNorthWorld map). In that area there are also disabled dead members of Pacer's gang here that are also clearly leftovers from an earlier incarnation of this sequence. There are also two dedicated dead off NCR duty troops presumably to be used during the shoot out with Pacer's men that are unused. In the final game all the NCR troops are alive when you get there.
Ruined store appears in Fallout: New Vegas.
There is a rare bug whereby the player is prevented from exiting the store. On the PC, this can be solved by accessing the game console during gameplay, by which you should be able to select the door and hereby input the command