If you don't use Alcatraz, the plugin manager for Xcode, you should.
When you install new plugins via Alcatraz (or any other way, really), you need to restart Xcode and then it prompts you with the following dialog:
If you're not paying attention, you may select "Skip Bundles" which will make Xcode not load any of your newly installed plugins, which is annoying.
Fortunately, there's a simple way you can solve this issue. Paste this in your terminal:
defaults delete com.apple.dt.Xcode DVTPlugInManagerNonApplePlugIns-Xcode-7.1.1
Press enter and restart Xcode. 🎉
Note the specific Xcode version at the end of that command — it's important!