It seems that part of the problem was that the VM memory was set to 4GB, which is all I have on the host. I scaled that down to 1788MB, and the VM booted, but all I get is a black screen.
You have to activate the processor’s virtualization extensions (VT-x) in the BIOS of your computer:
Less than 4 GB of memory is problematic for the virtual appliance. While you can tune it to require less memory, with the default settings it’s only happy with 4 GB and more.