It's usually a pretty good predictor of what your score would be if you took the actual test at the time you took the practice test. So, it you took Step 2 right now that is probably about what you would score. However, you have three weeks to study! You can raise your score a lot in that time. Take the other NBME sample test a few days to a week before your actual Step 2 test date...that way you'll have a good idea what you can expect to score on the real thing, and can delay the test if necessary. Don't be discouraged thought...you just started studying, and are probably a bit rusty on things you previously knew. You can change that score a lot over the next three weeks!
These things fluctuate significantly in how accurate they are. In general, people who score low on these NBME practice exam (such as yourself) tend to score MUCH HIGHER on the real CK exam than the practive exams indicated. I can not think of anyone with a low score on the practice exams who scored even lower on the real exam.
The medfriends score estimator only predicted a 161 for you. I think this is a very conservative estimate. It seems like this wavers a lot as well, as I know putting a 310 in a few months back would have led to a much higher prediction than 161.
There was someone from my school who said she failed the NBME exams 'utterly' and passed the real exam. I thought my real exam was easier than the practice tests.
Try to use the score report as a guide to where you are weak and focus on that. You still have more than two weeks which is a lot of time unless you're busy otherwise. I liked Deja review for certain topics with First Aid as my main source and QBank for questions.
You might want to take another NBME mini before your exam but, at the same time, it's late in the year (assumine you're an MS-4) and you don't want to let the test psych you out and make you push the test back. I had bombed two NBME exams and, even though I bought the 3rd, listened to a friend's advice not to take it. I had pushed my test back enough and had to take it when scheduled, so there was no need to take another mini and freak out with a bad score. You can take another if you want, but I wouldn't say it's critical.
Good luck! http://www.medfriends.org/step2_estimator/index.htm