Asus CN60 cant enter developer mode

Hi there, I am driving crazy with this Asus CN60 chromebox, I have already tried 9 keyboards (wireless and USB) some of them work because I can see that I can change the language when I try to boot with the reset button pressed with a paperclip, but when I try to enter developer mode with CTRL+D it just doesnt work with any keyboard and I dont know what else I can do.
I want to change the firmate of this device completely so I can install Linux Unbuntu, I dont want to keep anything of the ChromeBox.

Thanks a lot for your help.