how to open up a psdt file

A PSDT is a PhotoShop Document template. In older versions of PS changing the file extension to .PSD from .PSDT may be a solution for you. In newer PS versions, you're probably just running into your computer not knowing what application to use to open the .PSDT file by default. In this case, you can manually open the file in Adobe Photoshop. On a Mac, you can drage the file to your PS icon (maybe in your dock). Also, on a Mac, you can right click the file and choose Get Info. This will allow you to set the application you want to automatically open this file, and all future PSDT files. To make it a permanent setting, just make sure to click the button that reads, "Change All..."