See page 30 in the Manual for the Guide Number Chart for Manual Hi. The top chart is in meters, the bottom chart is in feet.
For Manual Lo you will have to divide the Guide Number shown in the chart by 4.
There should be a sticker with the chart on the flash unit as well.
It appears that Manual Hi is Full Power and Manual Lo is 1/4 Power.
Assuming you are using direct flash, you will have to find the GN in the chart based on your ISO setting and the Zoom setting of the Flash head. Then use the formula: Aperture = GN/Shooting distance (Flash to Subject distance) to determine the Aperture setting you will have to Manually set in the camera. When you use Manual Lo you will have to divide the GN shown in the table by 4 and plug that number into the equasion to determine the Aperture setting.
If you want to use an umbrella or other light modifier you will have to use a light (flash) meter or the Trial and Error Method to set the correct Aperture.
The beauty of a Nikon SB-600, SB-700, SB-800, SB-900, SB-910, or even an SB-400 or your built in flash is you can let i-TTL flash control do all the calculations for you instantly. Good Luck and Enjoy your Nikons!