You can use the 'locate command to find any file or directory. For it to work effectivley, you need to switch to su and run the command 'updatedb' ( without the quotes first. If you have never run updatedb, it may take a few minutes to complete. It will build a data base of all files and directories on your system. Once that completes once, try 'locate smbpasswd' ( no quotes ). If you have the file it will spit back the path to the file very quickly.
My smbpasswd file is located at /etc/samba/smbpasswd.
If at dought about what type of file a file is, use the 'file' command. A simple example is:
/etc/samba/smbpasswd: writable, regular file, no read permission
This what it looks like on my system. Hope this helps.