I was able to get this Plugin working by creating an x86 copy of the PaintDotNet executable. This requires the .NET framework corflags tool (available with Visual Studio or Visual Studio Express from Microsoft)
1) Open the Visual Studio Tools Command Prompt from the start menu
2) Change to the directory of your Paint.NET application (usually C:\Program Files\Paint.NET): cd C:\Program Files\Paint.NET
3) Type the following and press enter to create a copy of the executable: copy PaintDotNet.exe PaintDotNet_x86.exe
4) Type the following and press enter to change the copy to 32-bit mode: corflags /32bit+ PaintDotNet_x86.exe
5) You can create a shortcut to this 32-bit mode Paint.NET and the Twainable plugin should work
