The .tpr files are the Project Manger files where all their information is saved to.
To open an existing .tpr file, click the folder icon in the Project Manager's toolbar. This will allow you to navigate to a saved .tpr file and select it. The drop down arrow next to the folder icon can also be selected, which will show a list of .tpr files that have been saved from that instance of Toad.
To create a new .tpr file, click the icon that looks like a blank white page.