While our mindfulness instructor generally recommends sitting with a straight back because it can help facilitate focus during the session, this is not comfortable or possible for many people. If you find this position to be painful or uncomfortable, it may make it very difficult to rest your mind and concentrate. The most important thing is that you can sit comfortably for the length of time you wish to meditate. Some people prefer to sit with back support or lying down (so long as you don’t fall asleep :).
Finding a position that is comfortable for you is part of the process of learning to meditate. Listen to your body, if the instructions do not work for you, it's ok. Remember, they are only suggestions.
You may also want to try our “Mindful Walking.” Meditation is about cultivating a relaxed, focused, and accepting mind, and for some people, it's easier to do that in motion. Do what works best for you!