When I was using Virtual Box in Windows 7, I could choose how many CPU cores to assign to the guest OS. Now I'm using Linux, and when I installed Virtual Box, I couldn't find that option. System Monitor shows that when the VM is busy only one CPU core goes to 100% while the rest are near 0%. How can I make Virtual Box in Linux use multiple CPU cores?
I can simply set that in the VMs settings - change its details, go to the "System" tab, click on "Processor" and slide the slider to the right - done.
Did you check if the Host OS can actually see the correct number of cores?
You might want to check /proc/cpuinfo for that.
Picture of the settings tab / slider on OS X: