There's a slight problem though. The Live CD image listed on the official download page is actually a raw image to write into a USB thumb drive. VirtualBox does not support mounting a image as a USB drive. Therefore, we need to convert the bootable USB thumb drive image to another format. I converted it to a hard drive image for more flexibility.
Here's how (I found it in this guide):
$ VBoxManage convertfromraw -format VDI 2014-11-16-sage_6.4-debian_wheezy-live_3.0.5-1.img 2014-11-16-sage_6.4-debian_wheezy-live_3.0.5-1.vdi
By the way, 2014-11-16-sage_6.4-debian_wheezy-live_3.0.5-1.img is a 64-bit Debian image, which needs to be configured properly in VirtualBox.