After I upgraded my Macbook to OSX Lion, my NTFS-formatted USB drive would no longer mount... GREAT! %$#@
I use an free open source NTFS driver for Mac called NTFS-3G (You can get it from SourceForge here - grab the dmg file).
However NTFS-3G is built on top of a piece of software called MacFuse (part of the NTFS-3G installation) and unfortunately MacFuse only provides a 32-bit interface, whereas OSX Lion has a 64-bit kernel. I found the explanation of the problem here.
I was able to solve the problem when I came across a little gem on this site:
This guy has posted a custom compiled version of MacFuse, compiled for 64-bit.
I downloaded this guy's custom compiled version (look for the link to the zip file half way down the page) and just installed it straight on top of my existing MacFuse version. Then rebooted.
It solved my problem! I can now mount my NTFS USB drive and I have read/write access.
(N.B. I sometimes see an error when I connect the drive saying it can't communicate with the drive, but when you actually try to access the drive it works fine.)
If you have never had NTFS access before (e.g. you just bought a new Mac or just never tried it before) you have 2 options.
1) Install the open source NTFS-3G driver (from SourceForge as mentioned above) and then install the custom build of MacFuse on top of it. I'd recommend a reboot in between.
2) Or go for a commercial NTFS driver like this one from Tuxera.
Further reading: Check out this thread on macrumours.com for other people's experiences.