Marijuana is a schedule 1 drug under federal law. It is always illegal and there is no such thing as a legal "prescription" for it under federal law with a few exceptions (research etc), regardless of whether or not you saw Dr. Jerry Garcia Jr. and have a "prescription" for it.
BTW, no prescription for a controlled substance is legal under federal law unless the doctor has a DEA # and follows federal law.
Obama's policy merely says that the DEA and other feds won't generally won't prosecute "medical" marijuana users as they have "bigger fish to fry". You won't get that in writing and the policy can change at any day.