We have already learned how to create a bootable OS X Mavericks USB drive using a third party app, this time I will show you how to create one directly in Mac OS X.
You are trying to install or open an app that didn't come from the Mac App Store but you got a message saying App can't be opened because it is from an unidentified developer. This is how you bypass it.
You created your own movie in iMovie but you can't figure it out how to save the darn thing into a playable .mp4 file on your hard drive. I've been there, let me show you how to do that.