I'm using the 'Internal Network' type and 'Intel PRO/1000 MT Server' adapter type to simulate ethernet connections. I see it's learning ARP and LLDP shows active and connecting to my expected ports, but no luck pinging where I should be able to ping. Mainly, it has issues communicating with my other VMs. Interestingly too, I'm getting a license error with this VM as well even though I only gave the VM one CPU and 1024Mb of RAM as stated here. I had to download the OpenXen VM version to get the qcow2 file format then use the qemu-img Linux utility to convert it to a vmdk file (or VDI), which I could then boot from successfully. I did have to take a little bit of a round about way to get a FortiGate VM to even boot in VirtualBox. It'd be great to get one of those Fortinet Cookbook articles made up for this. pfSense and Cumulus were buttery smooth in importing their OVA file, booting up, attaching network interfaces, etc.
I've gotten very close myself, but I'm running into some strange obstacles. I'm looking to see if anyone has had any success getting an evaluation VM running in VirtualBox.