In article you see that "feature" file contains
ControlAssembly="SharePointSolutionPack, Version=1.0.0.0, Culture=neutral, PublicKeyToken=4a7cd02bdf107f7a"
ControlClass="Motion10.SharePoint2007.SelectItemsAction"
That means the code goes inside class SelectItemsAction within Motion10.SharePoint2007 namespace.
SharePointSolutionPack is name of Dll file, which, when compiled, you copy to C:\Windows\assembly
However you must also copy feature defintion to (default location): C:\Program Files\Common Files\microsoft shared\Web Server Extensions\12\TEMPLATE\FEATURES
A helpful resource for You
Anyway, what i wrote is not very helpful because i'm not going into details. See this step-by-step example on creating sharepoint button (exactly what you need) that uses Visual Studio extension that helps you develop features and deploy them (without copy-pasting i meantioned above). (You'll learn how to create features with that article)
It is important to learn about sharepoint features, because before i understood how to create features, it was hard to do anything in sharepoint (couldn't understand sharepoint articles, when i saw code, all the time question pops up - where do i put that code?).
Good luck!