Lumia535 uses the Denim system of Windows Phone 8.1, so there are only incoming call ringtones and reminder ringtones in "Ringtone + Sound".
SMS ringtones can be set in "Notifications + Actions".
First put the mp3 format ringtone you want to use on your computer into the ringtone folder stored on your lumia535 phone, then pick up the phone and set the ringtone in "Notification + Action".