Extract (as in open the .rar, pull the .esp out of it) then place the .esp, in the "data" of the SSE (Or whatever the fuck the 500th skyrim edition folder is called) folder to manually install it.

if using a Non-Playlist (Wabbajack) MO2 Install: Just click "Install mod" *or* drag'N'Drop the .Rar under "Mod Name".

if using a Playlist (Wabbajack) MO2 Install: Just click "install mod above", in a separator section thats either called ""Overhaul, Gameplay or Immersion"". (if it doesn't show in-game make sure you either reinstall the mod, or remove it and reinstall it again; As i've personally had this probelm with two race mods)