Enabling Time Machine for a USB HDD with NTFS
You can't backup to an NTFS formatted disk as stated below:


  Note: Every available disk that can be used to store backups is listed. If you’ve partitioned a disk, the available partitions are listed. Time Machine can’t back up to an external disk that's connected to an AirPort Extreme, or to an iPod, iDisk, or a disk formatted for Microsoft Windows (NTFS or FAT format).  If you select an NTFS or FAT-formatted disk, Time Machine prompts you to reformat the disk. Choose a different disk or reformat the disk in Mac OS Extended (Journaled) format. Because reformatting erases any files on the disk, only do this if you no longer need the files or if you have copies of them on a different disk.


This quote is from the apple support page for Time Machine  

You could always reformat the disk in Mac OS Extended (Journaled) format which would allow you to use it.