On this page:
http://wiki.xbmc.org/index.php?title=Plugins
"What about Linux? The SVN Repo Installer is not included?
If you installed the basic xbmc and xbmc-common packages in Linux (even from the SVN PPA), the SVN Repo Installer plugin does not come included. The fastest way to install it is to download it from the Google Code Project page (search code.google.com for the xbmc-addons project). Download the zip file into your plugins/programs directory (see below for where this folder can be found on your installation)"
--From what I've read the SVN Repo Installer is no longer valid, xbmc uses git
--There is no "plugins/programs" directory it's now all in the "addons" directory
Tried creating the "plugins/programs" directory and putting the SVN Repo Installer in it but xbmc does not recognize it
Found this thread explaining why:
http://forum.xbmc.org/showthread.php?tid=93619
Errors were not specific trying to install plugins that aren't addons as xbmc doesn't use plugins now it uses addons. Basically:
--Download awesome-pluggin-foo.zip
--Open System/Get Addons/Install From Zip
--Error: Plugin failed to install.
Hope that helps.
I would contribute to the wiki of all kinds of Linux/Open-Source projects if I had the time, but I work 6 days a week and I'm tired.