Well, I don't know much about that specific MB, but usually you have to press DEL, and when I say press I don't mean just press once and wait... usually, you have to keep pressing continually until you are inside the BIOS, but I guess some motherboards have some other ways to get into BIOS.
If during the startup Window (DOS) it says press SHIFT + TAB to enter BIOS, then I guess you have to keep pressing that combination of keys to enter the BIOS. You have to do that, otherwise you won't get into BIOS no matter what you do... so, keep trying...