I do not recommend modifying any of the mojoportal source code nor modifying any mojoportal tables. By modifying code you are making a fork and making it unlikely that you will be able to get upgrades without losing your changes.
You must be used to working with ASP.NET Web Site Projects which is not pre-compiled. mojoPortal uses the Web Application Project type where all the C# code is compiled into dlls. If you do not know how to work with that model I have no solution for you other than to learn how.
You simply create a class library project for a UserSignInHandler, you implement it and compile the project it produces a dll. Put that in the /bin folder and register it according to the documentation.
Hope it helps,
Joe