Maybe I'm confusing this a bit more than I thought. I'm changing this in TCP/IPv4 > Properties. When I have set IP in the past I've always had to set IP, Subnet mask, and Default Gateway. Of course I've never set them for VirtualBox. I'll try just setting the IP & see what happens...
If you did the DHCP setup part of the instructions, then you can just set everything to be obtained automatically. If you want to do the setup yourself, leave the subnet mask at the default (should be 255.255.255.0, I believe), set the IP to something in the 192.168.0.x range (that doesn't conflict with anything else), and set your DNS server to 192.168.0.75.
ZeroShell will act as its' own DNS server. It doesn't use TekSavvy's, it does the same thing as TekSavvy's.