dell xps 15 (l502x

Hi, i've tried to install El Capitan from my usb (used unibeast-->Legacy-->Intel HD Graphics 3000), but when i boot with my USB device, i receive this error: Operation system not found! My laptop is Dell XPS 15 (L502X). CPU Intel(R) Core(TM) i7-2620M CPU @ 2.70GHz, 2701 Mhz, 2 core, 4...