That's because it's formatted in the NTFS filesystem, which is native to Windows. Mac OS X can READ NTFS drives, but can not write to them.
Your options are as follows:
1. Format the drive in the Mac's native filesystem (HFS+), this is done in Disk Utility (Applications => Utilities). But bear in mind that this will make it incompatible with Windows, without a third-party driver.
2. Use a 3rd party filesystem drive for the Mac. NTFS-3G for Mac OS X is free, but some say it is unstable. I recommend Paragon's $30 NTFS for Mac OS X. Another option that chscag alluded to is to use OS X 10.6's native NTFS R/W driver, but that is still sort of in development and is not officially supported. If you Google NTFS Mac OS X 10.6, you should find tutorials to turn it on, but I wouldn't recommend it at the moment.
3. Format the drive in FAT32. FAT32 is an older filesystem, but is still useful for cross-platform compatibility. It has some limitations but works with both Windows and Mac OS X. The biggest problem is that Windows imposes a 32GB limit on partition sizes for FAT32... So, in that case, you'll need to format the drive under OS X in order to get a full 2TB volume.