Simplest thing on the Geck

Just look at the "Cell View" Window, here you have all the cells of the game. In the "World Space" Pop-up menu, select the type of cell you're lookin for (Interiors or WastelandNV are the most common). It may lag a little and then you will have all the cells corresponding to this world space listed below. Choose the cell where you want to place your item (example : there's a cell named "GoodSprings" in the "WastelandNV" worldspace) , it will load the cell in the Render Window. There, juste navigate to the spot you like to place it, and drag and drop your item there from the Object Window.
Render Navigation and Item placement is not simple at first and you should look a tutorials on the geck wiki