May be it's because the input for target and origin is a string and variable values are not properly interpreted.
Or it accepts only numbers.
You can make this work by "stringing" your properties, using the "str()".
example: str(Sprite.X) & "," & str(Sprite.Y)
However, this is very counter-productive as the events begin to look messy. Also, I find it weird that the Target is RELATIVE, and not absolute to the layout.
lunarray if you could change both "initial" and "target" to allow us to use any object properties (or, say, global variable), then the target wouldn't have to be relative to the tweened object. If you wanted that, you could simply use "Sprite.X + 100, Sprite.Y + 100"
Maybe 
Ashley can help us? :D
This is the nicest tween plugin we have here - it has all the powerful tweening features from Flash! The only thing holding me from using it is the ability to use an object property as coordinate or value easily!