Excel Add-In (Change existing add-in and program)

I have a program that was dropped in my lap that will need modification. However, even though I can install the program and see the Add-In in Microsoft Excel, I can't edit it. Also, in the source the .xlsm file doesn't appear to show any reference to the program being used. I can't seem to open the .sln file in Visual Studio. It just says VS 2013 can't open the file. I'll provide the entire source and bundled install. I need to figure out how to edit the application. What are the requirements and what do I need to do to edit it? That's the first milestone needed. I need to understand how to edit and change the program. I have worked a lot with MS Office, macros, VB.NET, C#, etc, but this thing baffles me. The first milestone will be $50. Once I get a firm idea of what needs to be done, there will be more changes needed a week or so from now. I will create different milestones for this work once I determine what changes need to be made. Hopefully you'll be able to help me then to both make the changes and get things rebundled. That will be additional milestones.