Question guys, what do you do when you have DCS World (1.2.4.12547), and don't have this magical "install" button.
A buddy of mine is updating his (slightly off topic) A-10C module, but it is a lesser version than Mondays update of DCS World. Again no visible sign of the "install" button, just a buy option.
Both of the currently available files (http/torrent) being 1.2.4.12179_180 version. Which produces the OP's error, but for A-10C.
-- Edit --
Part of the issue may be, his original purchase of DCS A-10C was via Steam. Which obviously isn't current and all that good stuff. So I'm uncertain if the being logged in part is failing, due to lack of correctness or similar. (?)