Hi Joel
Thanks for your answer.
In the rtmedia settings at buddypress, all buttons are "on".
I am able to mention the users, but i want to mention them also in a comment.
So when i click on a uploaded media from an user, there comes a lightbox for like and comment the picture, in this comments i would like to mention, and this does not work, but it should, after the new rtmedia update.
This was the update:
4.2 [Nov 17, 2016]
NEW FEATURES
Albums will now display media count by default
"@mentions" in comments added to lightbox and single media page
Added "Next" / "Previous" navigation links to single media page
Thanks for response 