For samba server to be seen by any other system, they need to be on the same sub-net. You have not mentioned the physical connections you are dealing with.
On Ubuntu, open a command prompt, and see if you can ping the NAS, or the Win7 machine. Any time you are dealing with servers, it is best to deal with fixed IP addresses, not DHCP as is the default on most new installs, including Windows. You will need updated hosts file on each system you want to join the windows shares. On linux, this file is in /etc/ directory, and is a simple text file called hosts. It is a list of host names to IP addresses. For example, an entry for your NAS might look like this '192.168.0.2 NAS Nas' To do a ping, then 'ping NAS' would do the trick.
IP connectivity has to work before you much with samba, and win config.
Next, post your /etc/smaba/smb.conf file. Please remove the comments, any line starting with # symbols. The outo generated ones are full of comments, and make it difficult for us to help. There are good tutorials on this board. You can fine a lot of basic help there.
W7 can be troublesome. microbucks has this thing they call 'homegroup'. That will only work with Vista and W7, not samba, or not your NAS. There are ways to get W7 to work, you have to set it up as if it were XP.
Test your IP connectivity, post your smb.conf file, and we'll go from there.