I'm not sure where you found this. But on X-Plane.org, in the download section, I found a cessna310R. unzipped it and installed it into x-plane11/aircraft/laminar and it went in without a hitch and runs like a champ. So, could you have it in the wrong place and not removed it. Or could you have downloaded a file for another version of XP (seems more likely). When I downloaded, I saw 4 different files for the C310. One for XP9.7, one for XP10, one for XP11 and one for multiplayer. Not sure how they differ, but if you have the file for XP9.7, it won't show up. Please make sure you have the correct verion. Other than that, sorry but I'm out of ideas.