Does MilHUD not work yet with 1.80? Or did I install the OXP incorrectly?
It won't support the new features, so OXPs adding waypoints or MFDs won't work with it, and you won't get the compass target label - but everything that was working in 1.77 or earlier should continue to work as before.
~/.Oolite/GNUstep/Applications/Oolite/AddOns is another place
Not on a Mac, it won't be. GNUstep is Windows/Linux only.
or you can get an idea from your Latest.log file.
This should find it, though. Post the file if you're having difficulty finding the install dir in it.