As Silox pointed out, you generally cannot just "open" an .EXE and see it's source code. Majority of .EXE files is created by compiles which take a given source code and convert it to a set of binary data comprehensive to CPU. This procerure is one-way unless you know _exactly_ how was an .EXE file created (which language and libraries were used, which compiler was used, the list is kinda long).
This plugin was apparently written in C#, so you might be able to get to the source code using
Dis#. But honestly, modyfing that plugin will definitely require some knowledge of C# or a similar language. Good luck