Windows Explorer won't return a string value for the path - for something that works in all apps, you should use the Windows API GetSaveFileName function (which displays a Save As dialog and returns the path - including filename specified).

However, I'm a bit confused as to how this is going to work.
The advantage of using the registry, or the user's homefolder, is that you can refer to the same location in the code for each user.
If you allow the user to navigate to a custom location for their user profile text file, you are going to need to store that location somewhere between application sessions... you might as well store the user's info... which brings us back to the original problem.
And there's no point in putting it in the text file, because it's the path of the text file you need to find when the addin loads.

???