The NTFS driver for Linux is complaining because the NTFS file system wasn't unmounted cleanly, and the only safe way to repair any problems that might have arose is to use Microsoft's tools to do so. You can borrow a friend's computer and run the disk error checker to fix the problem too.
If you can't do that, then you can force the drive to mount by typing the following. Make sure to replace the "X" in "sdX1" with the appropriate device name. In Ubuntu 10.04 LTS and Ubuntu 10.10, you can find the device name by going to System > Administration > Disk Utility, clicking on your external drive, and noting the name under "Device".
sudo mount -t ntfs-3g /dev/sdX1 /mnt/ -o force
You can get to the data then by going to Places > Computer, then double-clicking on "File System" and then "mnt".
While this will let you read and write to the drive, I would recommend copying your files off of the drive, and then reformatting it. This will write a clean file system to the drive. You can use Disk Utility for that as well.
Let me know if that helped!