At the moment, when you use an @Mentions tag then the namelookup dialog will appear and you can select a user name, for example @firstname lastname.
This works in the Connections Desktop Plugin and the Web UI.
However, if you post a status update in the Notes Plugin, this doesn't function like this there. It is a simple plaintext "@Mentions".
This is currently working as designed. This ER is to request that the Notes plugin use @Mentions functionality.