i have downloaded the following application:
"GetLevel - Revit 2008 addon utility - shows all elements associated with a level (S.Faust)"
but i still don't know how to activate it, can anybody assiste me ?
thank you

Copy the GetLevel.dll file to your Revit folder and then add the link to this dll file to your Revit.ini file. In the next Revit session you can run this function through:Tools > External tools > Get Level
 
(instructions were added to the download)