Well, the command you typed should work (except that you put two U in uusername) if you configured your Ubuntu share through Samba. It would not work if you used some other type of file sharing method, like NFS, but I believe the default method privileged by Ubuntu on the desktop is Samba.
Maybe your Ubuntu machine is not announcing its hostname on your network, and that is why you would not be able to access your shares list when typing
//ubuntu_machine. You may want to try using your Ubuntu machine's IP address. Instead of using a machine name, you would type for example:
sudo mount.cifs //192.168.0.100/shareforlder /mnt/share -o username=USER,password=PASSWORD
where 192.168.0.100 would be your IP address -- you have to substitute it by your actual IP address.
Try this, and give us some feedback!