To Open a Database in the main menu at the top is a option Open_Database, Press Return/Enter, This will open the Open Database Menu that will give a list of Databases which will be located in ~/.local/share/mpass/
To open a Database move the cursor to the database you want to open using the arrow keys and press Return/Enter.
You will be prompted for the passphrase and Press Return/Enter to enter the input.
To return to the main menu just press q to quit the Open Database Menu.