Open Visual Studio
Open FEFTwiddler.sln from wherever you downloaded the source to
In the "Solution Explorer" on the right, you should see the following:
* Solution 'FEFTwiddler' (2 projects) at the top
* FEFTwiddler right under it
Right click the FEFTwiddler and go to Properties
In the tab that opens up, change Target framework from .NET framework 4.5.2 to .NET Framework 4
Ctrl+S to save
In the top menu, go to Build > Build Solution
After it succeeds, right click the FEFTwiddler project (same thing you right clicked before) and choose "Open Folder in File Explorer"
In the Explorer window that pops up, go to the "bin" folder, then the "Debug" folder
FEFTwiddler.exe is in there. Give it a shot!