Do not constantly press the developer mode keys. The repetition of this can crash the BIOS and UEFI information causing your Chromebook to be a mess. I have been in your situation and here's how to do it. Disclaimer: You will have to get your hands a bit dirty and this process may revoke any warranty and static charges may damage the motherboard. Chances of this are low though :D. (These directions are only if you cant activate developer mode because it is blocked)