There are a number of methods you can use to select devices.
You can simply click the mouse on the check box of each device.
You can use the Filter method to display a group of devices, such as including all devices whose Group contains "Head Office". When the selected devices are displayed you can click on the Enable all button to check all the displayed devices. Note that the text you enter in the Contains field is case sensitive.
You can use your mouse to select a number of devices in the list using the standard Windows list selection methods. You can then use the right mouse button to show a context menu, and Enable selected to check the devices that are highlighted.