How do I Add a Password to a Zip File on a Mac?

Get ready for a trip back into the 60s...

Open Terminal (not kidding).
Use cd to go to the folder containing the file.
Type command below replacing archive.zip with the name of the compressed file you wish to create and file.txt with the name of the file you are compressing.

zip -e archive.zip file.txt

Enter password and confirm.
Exit Terminal and you should find archive.zip in the appropriate folder.

To compress multiple files in one archive.
Create folder.
Copy all files to folder.
Open Terminal and type command below.

zip -er archive.zip /path/file.txt