I have this exact problem, except I did not buy the Karts DLC. I did start noticing it shortly after the DLC came out though. I have tried:

1. Uninstalled ARMA 3
2. Deleted both ARMA 3 folders from my documents.
3. Deleted the ARMA 3 folder from C:\Program Files (x86)\Steam\steamapps\common\
4. Deleted ARMA 3 folder from C:\Users\Lang\AppData\Local\Arma 3
5. Deleted everything ARMA 3 from RegEdit
6. Re-downloaded ARMA 3
7. Installed ARMA 3, then ran it.
8. Click on Multiplayer, New, change from LAN to Internet.
9. Picked any mission, including editor (none of them work).
10. I get a blank role selection screen. If I hit ok, it says no players.

Screenshot of my problem.
http://grab.by/xyB4

This has happened for myself as well a group of people I play arma with, It started around the time of the Karts DLC release, of which not all of us own the DLC. Also in the first few days of the DLC release some of us where still able to view the slots when Local Hosting on the internet. I have done a steam verify on the game cache as well as a full re-install of arma 3 to try solve the issue, to no success. Hosting a mission in New > LAN the slots will be visible, but hosting a mission in New > Internet all slots and mission information will not show. This happens for both custom made missions as well as BI created missions.

This issue has be fixed for me since the June hotfix