I want to post this so it can get filed away in Google somewhere, so that if anyone runs into the same problem I did they can figure out how to fix it:
Back when I took the MPRE I hadn't yet decided which state bar's exam I would eventually take. Since I was still a California resident, however, I chose California to have my one free score report sent. But there was a problem: apparently you are supposed to register with the California bar as a law student at the beginning of law school. This creates a file for you, and must be done in order to be able to sign up for the bar exam later. You can, apparently, register after law school, but it costs about $50 more than if you had done it at the beginning of law school. (Of course, since I was at a school out of state I wasn't really aware of all this.) Also, it means that when your MPRE score shows up, California doesn't keep it - or at least not associated with you. Thus, when you eventually do sign up for the California bar, you have to get it resent. Or not...
It seems that all the scores are submitted to California after the MPRE administration in one big roster. A call to the National Conference of Bar Examiners who administered the MPRE can tell you which roster your score is on. Then you can call the California bar and they can look up your result on the original roster and associate it with your file, once you've finally registered with them. This is much better than having to pay another $15 to have the score report sent again.
So there you go. In case anyone happens to find themselves in the predicament I was in, hopefully this will save you some money and angst.