In the add to cart button behavior do not set a redirect for after the item is added in the cart.
After you have added the add to cart behavior, look for the php caode, it will start similar to:
<?php
// WA eCart AddToCart
if (isset($_POST["WA_Store_Cart_1_ATC"]) || isset($_POST["WA_Store_Cart_1_ATC_x"])) {
copy that if statement:
<?php if (isset($_POST["WA_Store_Cart_1_ATC"]) || isset($_POST["WA_Store_Cart_1_ATC_x"])) {
and use it around you message:
<?php if (isset($_POST["WA_Store_Cart_1_ATC"]) || isset($_POST["WA_Store_Cart_1_ATC_x"])) { ?>
Item was added to the cart message
<?php } ?>