Actually, it's very easy. No additional software is required. Just seven easy steps:

1. Attach your USB drive
2. Open the Terminal app (Command-Space, then type "Terminal", hit Enter)
3. Type or copy/paste this command:

   sudo sh -c "mkdir -p /mnt $(mount | grep ntfs | head -n 1 \
      | awk '{ print "&& umount " $3 " && mount_ntfs -o nosuid,rw " $1 " /mnt" }')"

4. Locate your drive in Finder
5. Drag/drop files there
6. Unmount the drive as usual
7. DONE!

The command breakdown, if you're interested:

1. mkdir -p /mnt creates a mount point -- a place in the file system where we will attach the drive
2. the mount command without parameters gives you a list of the currently attached drives
3. grep ntfs filters non-ntfs drives out the list
4. head -n 1 grabs the first line (we're assuming only one ntfs drive can be attached at a time)
5. the awk part produces two commands:
   • umount /Volumes/<name> -- unmounts the drive from its original place
   • mount_ntfs -o nosuid,rw /dev/<device> /mnt -- mounts the drive again, but this time in the read-write mode
6. now, the sudo sh -c "..." thing allows code execution with superuser privileges.

That's it.