Discover is not designed for scanning machines on a subnet, nor does it allow you to create a list of machines automatically. As an alternative, you can manually create a list of machines or use a third party tool to create a file with the list of machines and upload the file into Discover.
A third party tool can be used to discover all available file shares on a network. This tool can be used to create a file list that can be used to import the File Shares Target into Discover. http://www.download3k.com/SoftPerfect-Network-Scanner/Download-Free-netscan.exe.html
EXCEPTION: In the text file, list only share names and, optionally, scan depth, not username and password, for example, \\share\marketing,3.
Since the text file is not encrypted or otherwise secured, it requires the default username and password.