XMetaL Author does not implement an equivalent to the setInterval() method that most web browsers support (I assume that is where you got the idea).
Perhaps you could explain what you are creating in case I can think of an alternate solution. Knowing what you are building would also allow me to put in a feature request.
The closest I can get without using any 3rd party controls is XMetaL's GoToSleep() method. You might try putting that into a loop. However, that method is modal (stops the user from doing anything and displays an hourglass icon). I'm guessing that isn't what you are looking for.