Originally designed for the CodeWalr.us forum, the notifier evolved to support nearly all Simple Machines Forum (SMF) based forums.
It consists of a server application, written in PHP, which retrieves, parses, checks and caches the feed. And a client application written in C# with GTK#, supported by the most common Operating Systems, such as Windows, Linux and MacOSX, the latter two with the help of Mono, an opensource .NET framework VM.
The most important thing you'll need is the .NET Framework, with a version higher or equal to 4.0.
If you don't have the .NET Framework installed or if you only have an older version, then you can download the Framework version 4.0 directly from Microsoft. Now you only need to install the GTK# redistributable for Windows. You can download it from Xamarin's website.
After everything's installed, you can just go to the extracted folder of the notifier and double-click CodeWalriiNotify.exe.
To install Mono with GTK# on Ubuntu or Debian, run the following command as root:
# apt-get install mono-complete
Then just browse the the folder of the extracted notifier and run the following:
$ mono CodeWalriiNotify.exe
You can download the binaries of version 1.5 here.