Wireshark cannot see traffic from a VirtualBox guest on Windows 7
When guest OS is set up, a network interface is assigned to it.
Is wireshark listening on that interface?
In linux, there is an option to use "any" interface, which listens on all possible network interfaces, but I don't know if such option exists on the windows.
Here is explained that wireshark in windows has difficulties listening on loopback interface, the interface used when machine sends messages to it self.