Where To Find The Alignment Options Bar
The "Alignment Options Bar" is displayed when the "Move Tool" (V) is selected or in conjunction with "Paths" and "Vector Shapes".
Putting Alignment Options To The Test
Create a new document any size then turn on the rulers in photoshop by going to "View > Rulers" (Ctrl + R).
Position the mouse cursor over one of the rulers then drag whilst holding down the left mouse button towards the center of the canvas, when you get near the center of the document the guide should snap into place. If the guide doesn't snap into place make sure snap is turned on "View > Snap" (Shift + Ctrl + ;). Repeat the same steps for the second guide only this drag the guide from the opposite ruler.
Select the "Rounded Rectangle Tool" (U) and drag out onto the canvas a small rectangle. Select the "Move Tool" (V) then drag the shape into the middle of the canvas. The shape should snap into the middle of the guides.
If you would like to center layers without using guides then this is possible. First you have link the shape the background layer. Simply highlight the two layers then click the chain icon.
Now select the background layer then the "Move Tool" (V), to center the shape simply press "Align Vertical Centers" then "Align Horizontal Centers". Try pressing other buttons on the alignment options bar to get a feel for how it works.
Centering Objects Without Linking Layers
There is another method of centering objects within your document, this method doesn't involve linking the two layers together. Simply select the shape layer then select the whole canvas by pressing "Ctrl + A", select the "Move Tool" (V) then press the "Align Vertical Centers" button and the "Align Horizontal Centers" button. Magic! the shape is centered again.
Combining Selections With Alignment Options
Move (V) the shape from the center of the canvas then select the "Rectangular Marquee Tool" (M), drag a selection around one of the four boxes, the selection should snap into place.
Select the shape layer with the "Move Tool" (V), then from within the "Alignment Options Bar" experiment with the buttons and watch the shape dance around selection. With more than one shape you can accurately position each shape with no hassle at all.