If you are using Windows Vista, you probably have a problem with your Ink Services settings ( Ink Services are a method of dealing with tablet handling in Vista and Artrage uses them by default )
To disable Ink Service use in ArtRage, create a file called 'no_ink.inf' and put it in ArtRage's resources folder
e.g. c:\program files(x86)\Ambient Design\ArtRage 2\Resources\ <-- file goes here
The type of file doesn't matter, just the name. Once you've done that, start up ArtRage and you should have pressure again.
If that doesn't help, or I have misunderstood, let me know what operating system and version you're using, make sure you have the latest Wacom tablet drivers installed and I can help further.