For most installations, you should not have to use LiveUpdate directly. The definitions will be pulled by LiveUpdate on the client (this is not the same as the LiveUpdate Administrator which you find on the installation CD) but you will not really see it or interact with it.
Configure the SEPM server to pull definitons from Symantec (hourly / daily or whatever schedule you require). Put the clients in Pull mode and have them pull definitions from your chosen location.
If you have a very small number of clients it might be ok to check "use the management server", but mostly you will want to designate a client as a GUP. Pick a client at each geographical location (we use servers as they are available 24/7) and enter it in the LiveUpdate policy for that group. If you have a very large number of clients, you may need to set up LiveUpdate Administrator and use it to push definiton sets to your locations, but it can be hard work. The latest release (RU5) supports up to 10000 clients per GUP which should be enough for most environments. Stick with GUPs if you can.