It's only possible to add password protection to sub folder if the album is uploaded to jalbum.net.
Open the album you want to add password protection to and then go to Upload/manage.
Make sure your jalbum account is selected as account profile and press Connect.
When you are connected to the server you are ready to start adding passwords, but if the album is not uploaded yet, then you have to by doing that. Just press the Upload-button.
Click on the small arrow to show all the sub folders.
Mark the sub folder you want to add password protection to and click on the password protection-button in the tool bar.