Select it (use the arrow keys on your computer) and press "Enter".
If everything works properly, then El Capitan will boot. Once again, if you get a kernel panic/loading error when you try to boot your new Yosemite installation (or if the installation simply won't start within 10 minutes), you'll need to enter some boot flags.
To fix this problem, click on "Utilities" in the screen's top menu bar, and open up the Disk Utility app.
Then, in the sidebar of Disk Utility, choose the hard drive where you want to install OS X.
Then, once you've booted back into the Unibeast menu, input the necessary boot flags, and try starting OS X once again.
Check out our list of common boot flags and our guide to fixing boot problems with verbose mode for reference.
For El Capitan, however, we'll be using the new Clover bootloader (more on that later), which doesn't work with the MBR partition scheme. You will have to choose between two bootloader options: UEFI Boot Mode, and Legacy Boot Mode.
Here's a quick guide to differentiating between the two: the interface of BIOS is usually monochrome and entirely text-based.You should also choose Legacy Boot Mode if you're installing OS X on a hard drive with an MBR partition scheme (a.k.a the drive already has Windows installed on it) Next, you'll be asked to choose a graphics configuration.This step is only necessary for a few older graphics card that aren't compatible with OS X by default. For instance, if you're planning to install El Capitan on a computer with a NVIDIA Ge Force 9600 GT graphics card, you'll have to choose the "Inject NVIDIA" option here.If they were, but you still cannot boot from the Unibeast USB drive, unplug your USB drive, and go back to Step 1.Reformat your USB drive with Disk Utility and try again.In the worst case scenarios, instead of loading the Mac OS X installer, you may end up at a dark gray screen that tells you to restart your computer (a kernel panic), or you may end up with a small crossed-out sign (a loading error).