Yes - that error means that either the file name is incorrect or the path to the file is incorrect. My guess is it's the path.
From the top menu in Dreamweaver click on Insert, then click on Server-Side Include.
Navigate to the WA_eCart folder and choose file ACart_PHP.php. Double click on the file to insert it into the page. You will want to have your cursor positioned at the line of code where you want this inserted.
This should insert the file with the correct path.