The manufacturer field is a later addition to the software. It can be populated by simply adding information in Inventory Setup per screenshot below

Alternatively you can bulk update manufacturer information for all product records where the field is currently blank using the utility available from Inventory Setup. Right click anywhere in the Inventory Setup window, then navigate to Admin-->Utilities-->Populate Manufacturer Field

Reference Screenshot below for Utility Features:
1. Manually add manufacturers to the list as needed.
2. Manually delete manufacturers from the list as needed
3. Use the import manufacturers option to update from your own manufacturer list using a file in .tsv or .csv format
4. To populate the Manufacturer field for existing product records, use the Populate Manufacturer Fields option. Here's how it works!
- The utility scans each product description and searches for matches against the manufacturers listed in the window. When a match is found, the product's Manufacturer field is automatically updated.
- A report of all matched records is generated as a downloadable file, which can be reviewed in Microsoft Excel or Notepad

When Populate Manufacturer Fields is selected, click yes to populate blank manufacturer fields with values from the list:

Find the output file in your downloads folder
All products updated will show a match and 'updated' action

