If this is a self-managed client, it will download defs from Symantec by default anyway. If this is managed by a SEPM, then it needs to be told to do so (otherwise it will grabs defs from the SEPM itself).
The below article explains the LiveUpdate Settings policy options:
http://www.symantec.com/docs/TECH178257
In a nutshell though, you need to tick the "Use a LiveUpdate Server" checkbox, and choose the "Use Default Symantec LiveUpdate" option. Then have a play around with scheduling and you're away.