The LiveUpdate button is grayed out on an unmanaged client.
An unmanaged install package was exported without the correct LiveUpdate Settings policy so that it could update on its own.
The LiveUpdate component was left disabled in the LiveUpdate Settings policy prior to the export of the unmanaged install package used on this system.
Go to Start -> Run
Then browse to: HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Symantec Endpoint Protection\LiveUpdate
Before making any changes, make sure that you back up the registry.
On the right hand side, there will be a REG_DWORD value called AllowManualLiveUpdate
Right-click the value and select Modify
Change the value data to: 1
Once that has been done, the LiveUpdate button will be enabled and you can manually launch LiveUpdate.
It may be appropriate to export a new unmanaged install package from the SEPM that contains the LiveUpdate Settings you prefer so that the client will continue to update on its own without requiring you to manually initiate the update every time.