You don't actually need the contrib for this. The SME standard will do it for a single external POP account. The contribs help if you have more than one account to collect from.
In Server-Manager, go to the e-mail settings & choose 'change email reception settings'. Set retrieval mode to Multi-Drop & then complete the Multidrop settings.
Set Secondary Mail Server to the name of your external POP server & set the Pop User Name & Password. Configure how often you want to check for mail from the options given & away you go.
If you ever want to force an e-mail collection, log in to the console & enter the command /etc/startmail - this will immediately force an email collection.