Simply reply to the text message from your mobile phone starting your response with a Y if you are available or a N if not. If you have any further message for your manager, leave a space then add your message.
mAnagr.com will then receive your response, link it back to the message that was sent by your manager and change the message status icon on your manager's view to a green tick if your response starts with Y or a red cross if it starts with N. Any additional text is then displayed to your manager as a message from you. So you may reply "Y but I will need a lift" or "N sorry but my ankle is still not right. should be ok next week".
Your manager can then see on their view a list of all the messages they have sent for a match, the status of each and a list of all additional responses.