There a few ways to add plugins that are under development.
Specific git hash, against an accessible repo:
meteor add cordova:com.phonegap.plugins.facebookconnect@https://github.com/Wizcorp/phonegap-facebook-plugin.git#5dbb1583168558b4447a13235283803151cb04ec
From the local file system:
meteor add cordova:cordova-plugin-underdevelopment@file://../plugins/cordova-plugin-underdevelopment
When adding from the git repo, the plugin files are copied to the local build directory, and all the processors/compilers do their thing.
It seems when added from the local filesystem, the plugin isn't actually copied to your build. A symbolic link goes in instead, which mostly behaves the same way.
Only difference is in the generated XCode project.
The build doesn't create iOS binaries. Some licensing / tooling dramas, or something... Instead, an XCode project is created, and you fire up your copy of XCode, using your own developer certificates, etc, and compile that project yourself.
For the sym-linked version of the project, all the paths to your plugin's source files seem to be wrong. For "properly" copied plugins (eg. from the git repo), the XCode paths end up correct.
This screwed me up for so long...
When all else fails, fetch it from git.