You can manually add a 'generic windows file share' device to the core, which will cause your windows share to be mounted inside linuxmce, and the media should be scanned and aggregated by the UpdateMedia daemon. But normally this isn't necessary since the plug and play module is always scanning for new samba/windows shares on the net and asks you if you want to add them. Try double checking the username/password are correct. In the admin site, advanced, configuration, devices, under core you'll normally see the shares.