Mountain Lion has native read / write support, but it is not enabled by default.
Start off by editing your fstab like in any other linux/unix environment.
sudo nano /etc/fstab
Then add the following line to enable read/write permissions on your NTFS partition. Replace VOLUME_NAME with the name of your volume. You can find this in disk utility if you don't know it.
LABEL=VOLUME_NAME none ntfs rw,auto,nobrowse
Save by "control-x" to exit then "y" to save if you are not familiar with nano.
If you have a space in your VOLUME_NAME, you can escape it with \040.
You will need nobrowse parameter in order for this to work. This is so that it doesn't show up as a mounted drive on your desktop. Once you have edited your fstab, either reboot or unmount/remount the partition in disk utility. I would suggest reboot however.
In Finder, navigate all the way to the top with:
Shift Command C
or in terminal:
You should see your read/writable NTFS partition.
If you need quick access to the drive, create a symlink on your desktop:
ln -s /Volumes/VOLUME_NAME ~/Desktop/VOLUME_NAME