What is your version of Dreamweaver?
What version of eCart?
What other extensions, including the version numbers, are installed?
Make sure that you have a dynamic site defined in dreamweaver, and that you have a PHP page for the site open.
make sure that eCart is activated, try going to Insert -> WebAssist -> eCart -> Add to Cart Button, this will prompt to activate eCart.
IF the Add button is still disabled, try clicking the cart icon directly under the Add button this can trigger the add button to become active.