@Bryan Kelly, Please create a new MFC dialog-based project. Do NOT add ATL support.When the project has been created you should be able to build it and run it immediately and a dialog should appear as the main window as before. Close the application and return to the VS2017 IDE tht has the open MFC dialog-based project.
In the solution explorer pane select the project. Right click and pick Add New Item. In the next dialog under Visual C++ select MFC and pick "MFC Class" as seen here -
Add the new class to your project. You should now see
You need to make a few changes. First, change the Base class from CWnd to CDialogEx or CDialog -
After making that change you need to assign a Dialog ID -
Now accept the changes and click the OK button. MFC will create the header file (.h) for the new dialog, its implementation code (.cpp) and will create the dialog resource. If you go to the resource view you should be able to now see the new dialog and add controls to it from the toolbox.
Now it is up to you to add the code in your application to invoke the dialog by instantiating an instance of the new MFC class and calling its DoModal function.