1, give the win7 virtual machine installation enhancements, can not achieve otherwise
First select the Open virtual machine, then click on the "Device", you can see "Installation Enhancements", English is the "Insert Guest Additions CD image"
2, click into win7 virtual machine interface, open "My Computer" will show "CD drive", and then go in and check the "VBoxWindowsAdditions-amd64" this, choose a step by step installation until the installation is complete. Finally, let you restart the computer, restart direct it.
3, be shared with the virtual machine.
4, we've enhanced version features installed, inside the virtual machine you can directly see our shared directory. Here on the installation is completed. (Win7 need to open a virtual machine inside of sharing, in which a network.)