@mention(Activity stream mentions) is not working in the activity stream | Points to note

This functionality is configured by default. There is no additional configuration required for the @mention to work.

However there are few points to note

1) @mention feature works only on UI16
2) For the @mention feature to work, the field type of the journal fields should be of type "Journal Input" and not "Journal"

There is a PRB for issues with @ mention in the workspace for work notes and additional comments: 

The @mention feature does not work in the workspace for work notes and additional comments and clears the field content