If you're using NAT for networking with your VMs, you need to use port forwarding if you want to access a VM from the outside network. This is a quick and dirty howto on how to do this in linux.

Fire up a terminal, and enter these commands:

VBoxManage setextradata nvm3 "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/HostPort" 28080
VBoxManage setextradata nvm3 "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/GuestPort" 28080
VBoxManage setextradata nvm3 "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/Protocol" TCP

In the sample above, I'm forwarding port 28080/tcp from the host to 28080/tcp on the guest. My VM's name is nvm3.