It seems like different versions of DreamweaverMX might be behaving differently, but the instructions I posted above seem to work great on both my home and work copies.
It sounds like you still need to add the .tpl extension to your list of default editors in your DreamweaverMX prefs. Open your prefs menu in dreamweaver...
Now add the .tpl file extension to the list on the left, and associate it with Dreamweaver by adding dreamweaver to the editors list on the right.
Click done. You're done. Now you should be able to open those pesky .tpl files.
Getting back into x-cart dev after a long hiatus. Modded lots of x-carts from version 3.1.x to 4.1.x. Developer of ImageScaler mod, Pre-login per user coupon mod, Wordpress feed mod, DigitalSubscriptions mod, Phonetic bulk download keys addon for DownloadExpander mod, Serial Number Generator for ESD products, Custom CMS/LMS integrations, external products mod, and more.