The XSharp language ILSpy5 plugin was not working with the latest stable version of ILSpy : V5.02
This has been addresses and you can get that version in the Downloads/General/Tools section.
Unzip the DLL in the same folder as the ILSpy Binaries and you're done.

If you are looking to the ILSpy V5.02 binary, go to ILSpy binaries.
Now, when running ISpy, you can set the language as XSharp.

Don't forget that the full source code of the Plugin is available in the public XSharp Repository on Github; you can also view there the current state of developement of the tool ( What is working currently, Changelog, ... )

ILSpy is the open-source .NET assembly browser and decompiler, and as shown during the xBase Future 2018, you can now use ILSpy to view and decompile .NET assembly as XSharp Language.