If you wanted to move a sprite from Y position 0 to 100 over a period of 5 seconds, you'd have a timer marked at the beginning of the movement, then get the current Y position each pass through your loop by calling:
newY = LinearTween(0, 100, timer.TotalMilliseconds(), 5000)
Once newY = 100 or timer.TotalMilliseconds() > 5000, then your animation is complete. If you need to change direction in the middle of the animation, you'd set the start position to the current newY value. So basically, if you've decided that it takes a sprite 5 seconds to move a specific distance, then those are the values you'd pass to LinearTween. You just need to make sure your timer is restarted any time the animation (re)starts, and your start value is equal to where the sprite resides at the beginning of the animation.
As I mentioned before, I've modified Robert Penner's functions to better suit how I prefer to use them. You may have better luck using a straight port. This page details how they work:
http://upshots.org/actionscript/jsas-un ... ing-easing
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