The first thing to understand about adding actions to a monitor is to understand that some monitors are State monitors and some are Event monitors. State monitors keep track of whether the monitor is in a healthy state or an error state. For State monitors, you can choose to have actions run when a problem is detected, and then not again until it is fixed. Or, you can have it act like an Event monitor. Event monitors run actions every time they see something wrong. It is up to you.
With the differences in mind, the dialog below shows the action configuratino dialog for a State monitor. Notice that there is a set of actions to run when an error is detected, and a separate set for when the error is fixed (in the example below, the pager isn't notified when the problem is fixed).
On the right side of the dialog is the list of all existing actions. Using existing actions saves configuration time, and allows you to easily edit an action in one place and have all existing monitors use the new settings of the changed action. You can also quickly create a new action.
Actions are run in the order shown. You can click on an action in the list and press the up arrow and down arrow buttons to control its position within the action list.
Also notice that this dialog is where you create scheduled reports.