Help > Software Updates > Manage Configuration

Add Extension Location
But to my surprise, that option is no longer available in Ganymede! Instead you need to click "Software Updates" menu. Switch to "Available Software" tab, click on the "Add Site" button and choose the "Local" option button to select the directory where you have placed your plugins as an extension location. (see Ganymede screen shots below)
Help > Software Updates

Available Software > Add Site > Local

However, there's a catch to this approach, instead of running your plugins from the Extension location, Eclipse will copy your plugins to the features and plugins directory of Eclipse (which really defeats the point of having an extension location).
For a cleaner approach (than what was described above), Ganymede now supports "Dropins" (described in this article) which is sort of a hot deploy directory for plugins.
Just make sure your plugins are still compatible with the latest version Eclipse and you should be ready to go.
Now I'm off to explore Eclipse's latest features...
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