There are four actions call to action ,copy of action,call

to exesitng action,call to winner,split action just right

click on any one select it

just right click on the test pane select action if you want 
give action name

Your question was how to add actions in the test.
Are you trying to add a new action in the test itself or are
you trying to call an existing action?

1.If you are trying to add a new action: Alt I then N
or go to insert and select new action. It will generated
Action 2

2.Again the same procedure go to insert and you can select
Call to Copy of an Action or an Existing Action

Please refer to this VB Script. 

You can add new action programmatically also. 

' Dim qtApp, ActionContent, ActionName, ActionDescr, 
NewAction, SecondAction, script
Set qtApp = CreateObject("QuickTest.Application") ' Create 
the application object
qtApp.Launch
qtApp.Visible = True
qtApp.New

ActionContent = "Window(""Calculator"").WinButton
(""1"").Click" & vbCrLf & "Window(""Calculator"").WinButton
(""+"").Click" & vbCrLf & "Window(""Calculator"").WinButton
(""+"").Click" & vbCrLf & "Window(""Calculator"").WinButton
(""="").Click"
ActionDescr = "A new sample action for the test."
ActionName = "Action" + CStr(qtApp.Test.Actions.Count + 1)

'Add a new action at the begining of the test

Set NewAction = qtApp.Test.AddNewAction(ActionName, 
ActionDescr, ActionContent, False, qtAtBegining)



For Attributed like ActionContent, please read all text 
from any Text file in which you have already written some 
code. 


With this help you can generate new QTP Script pretty 
quickly.