Select the cube you want to texture.
Name the selected object “Box” (double-click on the cube in the scene/hierarchy view in the upper right and type in a new name for it)
Now is a good time to save your Blender file, give it a name like “box.blend” and keep all your files for this model in the same folder (to see these files in Unity, keep them all somewhere under “Assets”, e.g. “Assets/VIEN/Props”)
Switch to Edit Mode (The mode selector currently shows “Object Mode” in the upper left)
Switch to Edge Select Mode (Three buttons immediately to the left of the mode selector: vertex, edges and polys)
Multi-select (hold SHIFT) the “seam” Edges of the Box (for a fold-out cube)
Edges→Mark Seam (Menu above the view port)
Select the UV Editing tab at the top of the screen.
Select your object again in the 3D view (press “a” to select the whole object)
If a UV image appears in the UV Editor window on the left, showing how all sides of the cube fit onto the image, then you can skip the next step (the UV coordinates already exist, so no need to unwrap again, unless you want to try)
Either window: UV→Unwrap (you should now see the fold-out texture from the cube appear in the left UV Editor window)
UV Editor Window: UV→Export UV Layout.. (you should save it in the same folder as your 3D model)