It's not impossible to get into med school without altruistic activities, but a lot of schools won't consider you if you don't have them. You will probably get some forgiveness if you were working 40 hour weeks, but not so much with 15-20 hours of work per week. Other activities aside from community service can demonstrate altruism. If you are an intense althlete on the school's team, if you help to support your family, or are exmilitary, are some examples of ways you can show you are acting beyond your own self interest.
One of SDN's regular adcomm posters feels that if you can't find an extra two hours per week to volunteer, your application goes way down in her estimation (because young folk can go without a bit of sleep). You might consider getting something going so it will be on your application, then you can update schools in periodic letters as to additional hours acquired, hoping it will sway them positively.
