The cartridges are pretty useless now. All of the NetJet game files are actually stored on their servers. The cartridges are just used by the software to check if you actually bought it, in order to let you download the game. What's neat is that even though NetJet doesn't exist as a website anymore, you can still run and play your games just fine by using the executable in the NetJet games directory (the MBXP game folder is
really similar to MBG's folder). The cartridge check is only performed when you're downloading the files for the game. The NetJet software just acts as a wrapper for the game, which can be run pretty much on its own.
Anyway, that was just a little bit of info I thought might be helpful. Since you don't appear to have the game files on your computer,
here's a link
to my MBXP game folder. Just place that into your NetJet\Games directory and run MarbleBlastXP.exe. If you have the controller plugged in, the game should start up and work just fine.