First of all unless you use the Captive tool to load the NTFS drivers and use them, you can do serious damage to your NTFS partition if you try and write to it. Using captive is easy enough with Knoppix 3.4. Just navigate through the "Knoppix" menu and run the Captive tool and let it walk you through the set up proces.

Secondly you will still need to grant write access to your NTFS partition after you mount it. You can do this by right clicking on the drive icon on your desktop and changing it to read/write.

After doing these things you should be able to write to your NTFS drive without any problems.