If you intend to do it manually, use either CTRL+ENTER or SHIFT+ENTER to create linebreaks. Using code, you should be able to use something like this:
[tt]me!txtSomeControl.ControlTipText = "This" & vbcrlf & _
"is" & vbcrlf & "a" & vbcrlf & "test!"[/tt]
Roy-Vidar