If you want to enable MAC address vendor listing with NetScan, you will need to download a text file listing all the manufacturers and save it to the same folder as the netscan executable.
Download this oui.txt file and save it with the netscan.exe executable. This is a "sanitized" version of the file and should download faster than the original.
The in the Program Options for netscan (Ctrl+O or Options...Program Options), on the Additional tab, turn on "Lookup network card vendor".
FYI - the original file can be found here - http://standards.ieee.org/develop/regauth/oui/oui.txt