Replace your code with this:
function WaitAliasChildExample()
{
// Specifies the Alias object
var AliasObj = Aliases["notepad"];
// Checks whether the Font window has appeared within 10 seconds
if (AliasObj["WaitAliasChild"]( "dlgFont", 10000 )["Exists"] )
Log["Message"]("The Font window has appeared")
else
Log["Error"]("The Font window has not appeared.");
}
Note that on the line generating the error, I got rid of the open and close parenthesis on the Exists property. It's a property and not a method and so it does not get that empty paren call.
This looks like a minor typo in the help files.