Simply click on the browse button and select multiple files using the CTRL key or SHIFT key and mouse. You can also select multiple files by clicking on the Browse button (next to the File(s) to Encrypt field). You can manually type additional files for encryption by separating the name/path of the additional files with a semi-colon. You will see that the name/path of the active file is populated in the 'File(s) to Encrypt' field. You will then be presented with the Encrypt File(s) dialog.
With the desired file as the active file in the editor go to the File Menu and select Encryption : Encrypt File First you will need to browse to and open the desired file using the File : Open dialog.
