Try F8 after the BIOS post (splash screen) and you should be able to select your boot device from the advanced boot menu.
I did just write a length post about usb keyboards during boot then i searched Asus K53E and it is a laptop you are refering to?
If so makesure there isnt a function modifier key which you need to hold down while also pressing one of the F Keys (which should be F2 according to the asus website page for the K53E)
i hope this helped.