Try the free and open-source NTFS-FREE if you don’t want to spend any money on this feature. There are quite a few solutions for writing to NTFS file systems on a Mac, many of which are paid applications. Unfortunately, Mac OS X can only read this partition out of the box, not write to it. Your Windows partition appears under Devices as BOOTCAMP on Mac OS X.
Write to Windows NTFS Partitions From OS X HFSExplorer is a free tool for accessing Mac partitions from within Windows, but it’s read-only so it won’t help you here.
