In case you did not auto-mount VBox shared folder then you may go to Device->Shared Folder Settings, edit the shared folder, and enable Auto-mount. You may also check "Make Permanent" if you want the mounting to stick on reboot.
In case you did not auto-mount VBox shared folder then you may go to Device->Shared Folder Settings, edit the shared folder, and enable Auto-mount. You may also check "Make Permanent" if you want the mounting to stick on reboot.