Plugged in Passport to my Mac os 10.5.8 see icon but no install info

If that drive is not a passport for Mac, you will need to format the drive to make it fully compatible, once you do that time machine should ask you if you want to use the drive for backups.

If you want to use the WD Smartware backup software, you try downloading that from the web site and installing that on the computer. Again if the drive is not ready for Mac, you will need to format it.