If you're referring to the marksmen dlc, the dlc hasn't been published yet and won't be until April. Some content is available in the dev branch but as for the normal version of Arma 3 no new content of that dlc is playable so far.

If you purchased the dlc pack you'll still see the notification but just ignore it for the moment. I suspect that you'll unlock all content once the dlc is actually released.

If you're not referring to the marksmen dlc, make sure that you got them selected (steam, library on the top right click on detailed view, you'll then see the list of all dlc you own and whether you deselcted them).

The artwork is however the one that gets it right though. I'm not sure if I made that clear. The ingame 3d model has the "wrong" sight and missing valve.

The sight should indeed be very similar to the one on a mp5.