Installing Excel-Dna

With the package manager console:

Install-Package ExcelDna.AddIn

Add a basic function:

using ExcelDna.Integration;

public static class MyFunctions
{
    [ExcelFunction(Description = "My first .NET function")]
    public static string SayHello(string name)
    {
        return "Hello " + name;
    }
}

Compile and run to use it in Excel:

=SayHello("World!")