Error when running Rails Server on a local copy: Could not find mysql2-0.2.6 in any of the sources. Try running `bundle install`

I just forked a copy of an app on github and saved it on my computer. When I attempt to run the rails server in the application directory, I get the following error:

Could not find mysql2-0.2.6 in any of the sources
Try running `bundle install`.

Earlier today, however, I installed MySQL 5.1.53 for Mac OS X and supposedly inst开发者_如何学JAVAalled it...at least I'm quite certain I did.

Why does terminal come up with this error message even though I'm quite certain I have MySQL already installed on my system?

It's not complaining about MySQL itself, it's complaining about the missing gem.

Just do what the message says, run a bundle install to download and install the gem.