Drawing with the Rectangle Tool (M) is a basic part of building a Wireframe. Even when you are going to be making Rounded Rectangles its still better to use the Rectangle Tool. I'll show you why as I build the header. Draw a rectangle 960px Wide and 120px Heigh, that is about 40px from the top of your Artboard, and on its sides touches your guides. After selecting the Rectangle Tool (M) you can click anywhere in the document and a Rectangle Dimension Dialogue box will come up. Or draw a rectangle with your mouse and use the tools below to manipulate your rectangle if it wasn't drawn at the correct size.