The term 'AssembleMessage' is not recognized as the name of a cmdlet

Export-Message $MessageID | AssembleMessage -Path c:\Temp\message001.msg

If I use this command in EMS it'll work fine. If I use it in an ISE, such as Powershell ISE or PowerGUI Script Editor, I get the command not recognized error. I have the the Exchange snap-in loaded (add-pssnapin Microsoft.Exchange.Management.PowerShell.E2010) what else must I load to make AssembleMessage work?

Thanks

Chad