I've downloaded the TBB but it won't open because "TorBrowserBundle_en-US" is from an "unidentified developer". How do I solve this?
With some security configurations Apple's operating systems will refuse to run any application they haven't specifically vetted by Apple Corp.
You can get around this easily.
You can choose to override your security settings and open the app anyway.
In the Finder, locate the app you want to open. Most apps can be found in the Applications folder. Press the Control-key and click the app icon. Choose Open from the pop-up menu. Click Open.
(The Mac OSX 10.9 support document has the same suggestion)
Reminder: any executable file from the internet is subject to tampering. Be sure to verify the gpg signature of the Tor file you downloaded before running it. Apple's Gatekeeper program (the feature that blocks the initial running of Tor) is designed to help ease the process of verifying binaries for most users but for third parties who are not registered (like the Tor foundation) it results in extra hassle.
Just add to reset the policy to "Apple only" (or whatever it was) afterward. The other answer is also wrong, it is fully possible to run what you develop with Xcode and also the Android Studio. Use System Preferences is a simple and easy way to do this. And of course you can download a key and patch it in manually also - but use the method that Apple endorses. The reason for this is to enable them to halt you should you try to launch something that someone else has managed to download on your Mac, and even created an icon - this is NOT MICROSOFT WINDOWS and we don´t want those policies!