After upgrading my Macbook Pro to OS X Lion I was no longer able to mount any NTFS drives, not my USB drives formatted for NTFS nor even the Macbook's own BOOTCAMP partition. I could see the drives in Disk Utility but if I tried to mount them I got the error message "The disk “BOOTCAMP” could not be mounted. Try running First Aid on the disk and then retry mounting."
I tried lots of all of the third-party extensions I could find, but they all failed. I found the solution on Paragon-Software's website. I fixed the NTFS mounting by deleting the folder /System/Library/Filesystems/ufsd.fs and rebooting.
Let me know if it works for you.