Prior to Wireshark 4, you would find the manuf file in the Wireshark program directory. The manuf file was a simple text file containing OUI information used for MAC address resolution.
Now the manuf file (along with the enterprises and services files) has been replaced with static internal binary data.
If you are one of the folks who altered the contents of the manuf file to have customized values, you can still do so because manuf file can still be read from profile folders.
Step-by-Step
Step 1: First, you need to create an manuf file using Tshark. The syntax is tshark -G manuf > manuf.
Step 2: Now edit the text file as desired. Do not change the name - it should simply be called "manuf."
Step 3: Determine which Wireshark profile you want to use the manuf file in and copy the file to that location. (Alternately, navigate to that profile's directory and run Tshark from there.)
Now, when you are using that profile, it will pull the manuf file in the profile directory.
One additional change is under the Tools menu item. Click on MAC Address Blocks to see the master internal list of OUI values and vendors. This won't read the customized manuf file you added to your profile, however. If you want to see that, just open your manuf file directly.
Enjoy!