How do I get Time Machine to Show in Notification Center?

You don't need to add Time Machine to the list. Along with all other OS X system notifications, they are shown in Notification Center without any user intervention required to activate them. Conversely, you can't disable them either. This is the same with battery notifications, disk ejection notifications, etc.

If you want to disable Time Machine notifications, set the flags value for _SYSTEM_CENTER_:com.apple.TMHelperAgent in the app_info table in ~/Library/Application Support/NotificationCenter/<udid>.db to 12609 or 13639.

If you want to customise it further, change one of the other entries for Notification Center, then read the flags value for that app and save it as the flags value for TMHelperAgent.

To disable Time Machine notifications on Yosemite, first find the Notification Center database by running:

cd `getconf DARWIN_USER_DIR`/com.apple.notificationcenter/db

You'll end up in a directory containing a SQLite database called db. In this database's app_info table, set the flags value for _SYSTEM_CENTER_:com.apple.TMHelperAgent to 12609.