Not sure what the screen shows on the next page - but all you actually add to the media server is the serial number - as long as the serial number is installed and you then install the agent software on the MAC itself it should let you do backups. No extra software is installed on the media server although the serial number's presence does turn on some options in the BEWS console.
EDIT: Important thought I just had - the current MAC agent does not support Snow Leopard - support will be offered by a Hotfix for the BE2010 version at some point in time after the BE2010 release - as such I hope you are running Leopard or earlier
