There are two main reason why you would want to focus bracket. The first is Focus Stacking where you achieve a greater depth of field by combining multiple images with different focal points. The second is to ensure that you receive a single final image with the desired focus point.

You are using this for macro photography so I am assuming that you would like to stack images in order to achieve a greater depth of field.

The Nikon D90 does not have the ability to bracket focus. This is one area where many point-and-shoot or pocket digital cameras have a slight advantage to most DSLRs. You can achieve a similar effect simply by putting your lens into manual focus mode and racking the focus back and forth for multiple images. You also can put the camera into auto mode and simply select a different focus point over multiple image captures, but in practice this will be slightly cumbersome.