With BES version 4 you could refresh the list of available users by simply closing and reopening the desktop manager. Not so in BES 5. Here is what you need to do.
Check to see if the Blackberry Mail Store service is running. To update the list on demand , expand BlackBerry Solution Topology then Component View then click Email and click Refresh available user list. This will update your available user list and you can then add your new user normally.