The package you download can update both FCS and FEP 2010.
To manually start the signature update from the client software run
In FCS you should therefore run
C:\Program Files\Microsoft Forefront\Client Security\Client\Antimalware\MpCmdRun.exe -SignatureUpdate
In FEP 2010 you should run
C:\Program Files\Microsoft Security Client\Antimalware\MpCmdRun.exe -SignatureUpdate
On Windows 7 you should run MpCmdRun.exe with elevated privileges or from an elevated command prompt.
Don't forget that because the path to the executable contains spaces you'll need to use quotes if run in a Run Command Line step in a Task Sequence.