Time machine doesn't recognize my external hard drive

I have a MacBook with Leopard and trying to use time Machine to back up to a Seagate Backup Plus external hard drive. The Seagate says it can be used for both Macs and PCs. I connect Seagate to laptop and the icon shows on the desktop and it shows up in Finder, but the Seagate does not show up in Time Machine Preferences in order for me to choose it as the drive for the backup. Any ideas?