make sure that the site is defined properly for dynamic development, see the following tutorial for details:
http://www.webassist.com/tutorials/Define-a-dynamic-site-in-CS5
make sure you have a PHP page open that is saved to the site. If a PHP page from your active site is not open, the eCart floater will not be active.
in the eCart floater window, you may need to click the cart icon next to the "Add eCart to this site" that should trigger the add button to be accessible.