There are various ways to mount your stashes. You can use the tray menu, click on the "Mounted"-checkbox of the stash in the manager window or let it automatically mount when MEncfsM starts or the stash becomes available.
The stash will try to use the password saved in the keyring when set and mount without any user intervention. If there is no password available or the mouting fails, the mount dialog will open and inform you of the reason.
You can cancel the mounting by either clicking on "Cancel" or by hitting escape on your keyboard. The password will be saved in the keyring if the stash was successfully mounted and the option is set.