After creating & slicing the layout in Photoshop to export the .psd file as a html file you have to use ImageReady. In ImageReady you can also define the rollover states of a button.

Initialize ImageReady and open the file index.psd from template/psd (choose File -> Open from the main menu):

The opened file will look like this:

The ImageReady Tools palette and the others palettes are a little bit different from Photoshop. The most important difference is that in ImageReady you have a palette exclusively for slices. This palette is named Web Content. If you don't see it you can open it from Window -> Web Content.

As you can see the Web Content palette looks very much like the Layers palette but instead of displaying layers it shows the slices and their name.

To select a slice you can go and click over the slice in Web Content palette or you can select the Slice Select Tool from Tools and then click over a slice in the layout: