The four machines found should relate to the host machine, the DHCP server, the metasploitable server and the backtrack attacker. In order to know what the problem is, we need to find out which is missing.
Can you let me know the IP addresses of the machines it finds?
The next place I'd check is the networking setup in virtualbox. first, make sure all the machines are connected to a 'host only adapter' rather than any other of the networking methods, and then check the network setup in virtualbox. Go to file > preferences > network. There should be at least one network listed there (if there's more than one then make sure all the virtual machines you've created are attached to the same one) then click on the screw driver and post the settings in the dialog.
Let me know the info from here and I'll see if I can work out what the problem is.