when i created shared folder getting error , The VirtualBox Guest Additions do not appear to be available on this virtual machine, and shared folders cannot be used without them. To use shared folders inside the virtual machine, please install the Guest Additions if they are not installed, or re-install them if they are not working correctly, by selecting Insert Guest Additions CD image from the Devices menu. If they are installed but the machine is not yet fully started then shared folders will be available once it is.
Assuming you have not installed the 'Virtualbox Guest Additions' (you do not mention if you have done so in your question) you simply need to install them reboot the virtual machine and then sharing folders should work.
Source: http://www.virtualbox.org/manual/ch04.html#additions-windows