Possession of cannabis is still a violation of federal law. As of March 13, 2013 patients with a valid physician's certification for the medical use of cannabis may possess up to a 60 day supply in the Commonwealth. Expect this to change when the dispensaries open.
A good summary on medical cannabis for Massachusetts is on the NORML website.
The term "marijuana" should be avoided because it carries a negative connotation. A change in terminology promotes a change in attitude. So try using the term cannabis instead and see how people react. Remember, the sale and distribution of cannabis is still a violation of federal law. Attorney Russell is a member of NORML, the National Organization for Reform of Marijuana Laws. The NORML law summary for Massachusetts is here. http://norml.org/laws/item/massachusetts-penalties-2
You can learn medical cannabis in Massachusetts from the government too.
A government source for patient and caregiver information is below.