well your using the wrong handler for a start
on mouseenter me
on mouseleave me
Try this page on my site - it deals with backdrops which are actually just overlays as far as lingo is concerned( Just replace the word overlay where you see the word backdrop)
thats not exactly what your after but it'll have to do for now - it shows you how to create an overlay andmove it about. with this behavior I would create an overlay then make it visible when mouseenter and invisible when mouseleave.
for helpful tutorials try this site - its really clear
and heres my links page - there are other helpful tutorials out there
oh and btw , your book, well it may come in handy pretty soon as its coming up to bonfire night.
HAIRYBOBBY - why didn't I choose a really cool name like the vulcanpimp
new vector shape behaviors here