Looking into this further I must admit the first solution I suggested is not too user friendly since it involves installing several requirements first.
I don't really have a fix at this time, your best bet is to wait for next release, which will probably come with a static build of 7-Zip included.
For those who need it, Weltall has kindly provided a
static build of 7-Zip / p7zip (also
mirrored on dl.megaglest.org). This should be version 9.20.1 which is the latest stable 7-Zip release at this time. Another option is to install
Keka which usually includes a somewhat current static build of p7zip (the executable file is called
keka7z).
But having 7-zip is just part of the game. You need to make MegaGlest find it. I assume (untested) this can be achieved by editing glestuser.ini (which I assume is stored at
/Users/YOUR_OS_X_USERNAME_GOES_HERE/.megaglest/glestuser.ini) and adding the
FileArchiveExtractCommand configuration option there, including the full path to where your p7zip binary is located. For example, if you installed Keka in Applications (such as by dragging + dropping the DMG there), you'd need to put something like this into glestuser.ini (don't hold me liable on this):
FileArchiveExtractCommand=/Applications/Keka.app/Contents/Resources/keka7z
But really, anybody who has (even less than myself) no idea what I'm talking about, should just wait for the next MegaGlest release to download techtrees from the Game Mods menu.