When you create your element set the width and height of the application to that of what you require for the website. Then fill the app with the tab navigators width and height set the same.
Then when done the result of a flex app (usually and I imagine so co this seems like a small requirement) will be a swf. You can embed this as you would any swf like from Flash Professional for example.
Ps. Yes File > New Project.
thx I created this element with smaller size and transparen background:
But the finall result is horrible:
on the white background: http://img4.imageshack.us/my.php?image=09012009160907.png
on the yellow background: http://img4.imageshack.us/my.php?image=09012009160937.png
I can't make transparent background in Flex ?