Thanks, guys. Well, I can access the FOG configuration just fine (by entering its IP address into a browser). My concern is that with this problem, I will not be able to register any additional hosts because they will not boot to PXE.
When I installed the FOG server, I chose to make the server act as a DHCP Server. In VirtualBox, I configured the FOG clients to have bridged network connections, and that allowed PXE boot to work. I made absolutely no changes to the VirtualBox settings before PXE stopped working; the only thing that happened was rebooting the computer.
I've just run a "ps -A" command in the Terminal. These FOG processes are running: "FOGImageReplica[tor?]", "FOGMulticastMan[ager?]," and "FOGTaskSchedule[r?]." So it appears that FOG is indeed running. PXE boot is just not working.
Edit: Sorry, I neglected to mention that I configured no TFTP server. I just find it strange that FOG worked perfectly until I just restarted the computer.