i created a password-protected disk image, but don't understand how to add files or how to protect the files??
Mount the disk image (it should show on your Desktop) and drag your files onto it.
The cursor will change to a green + symbol and a dialog window will open showing the data transfer. Files will be copied (not moved) to the disk image.
Mounting the disk image is done by double clicking on it, or selecting it then Finder -> File -> Open, or Select then Command Open, or righ-click (control-Click) -> Open
Retrieving data ...