Luckily, I found out how today, and I'm happy to share the information.
1. Go
here, and scroll down all the way to where it says Center Pan Remover, and then download it.
2. Click on 'My Computer' Icon (Or wherever you saved all the Audacity stuff), and then click on 'Local Disk (C: )'
3. Click on the Audacity folder, and then click the plug-in folder, and simply place the file that you downloaded into the folder, and the feature is now usable.
4. Load a song, Highlight it with your cursor, and go to the 'Effects' section where you'll see Center pan remover.
5. Click the center pan remover button, and then your done.
Glad to help!