virtualbox.org

I've got Ubuntu 8.10 running on my Gateway with an Intel Core2 Duo. I just upgraded to 3.0.4, but I can't set my guest SMPs, either for vms I already have or newly create. My vms run fine, but I'd like to give them more than one processor. Does anybody know what's going on?

If it is grayed out then VBox does not see the VT-x/AMD-v flag for hardware-v.
Look in your bios and turn it on. If you do not see it then SMP will not work for you.