While there are plenty of tutorials out there regarding dashed strokes in photoshop, this is a little trick i do when I need a quick dashed line to use in web layouts:
1. Open a new document in Photoshop. The size depends on the length of the desired dashed line.
2. Make a selection with the Rectangular Marquee Tool
3. Take a screen capture of your selection.
4. After you have screenshot of your dashed box (or circle/oval) you can bring it into your layout, Repeat or layer as necessary to get the desired length or effect.
Not the only way to get a dashed line in Photoshop, but quick and easy!