boot from usb acer chromebook 315 won't boot from USB

[bjorkostof]

Hello there.After doing a lot of research myself i'm stucked.My chromebook don't recognize usb when i press ctrl+L 

What i've tried did do far : 

put in developer mode

after start up press : ctrl+L  got black screen with only the installed OS,no option to chose other nr

ctrl+alt+ right arrow login = chronos   sudo=bash

from there execute the cmd = crossystem dev.....

so i know i do something wrong,really want to get on with this so i hope someone can help

greetings,bjorn