There are two icons in the upper-left corner of the gameplay screen. The first is the special operations bar. You can click on it to expand and collapse a set of icons for each of the special operations available to your general. Click on any special operation that is not grayed out to use it. The icon then appears gray and the red bar beneath it begins to refill.When the bar is full, you can use the ability, provided you have the army experience to pay for it. The group bar is located to the right of the special operations bar. Whenever you assign a group to a hotkey, it appears in the unit bar. As with the special operations bar, clicking on the icon will expand or collapse the entire group bar.
Source: Axis and Allies Official Manual