Hi Wayne,
Your diagram shows the right principle for switching the multiplexed segments. Just a couple things to note.
Are you using the BC550 for both the top and bottom transistor? The BC550 is an NPN, so if you use it at the top, you provide 5V to the base, and the emitter will be about 0.7V less than that, so around 4.3V. It sounds like you were expecting 12V and sized your resistor accordingly, so maybe that's why you aren't seeing enough current. Try something like a 330 ohm resistor.
Note also that the top transistor will have a drop of almost 8V and have something like 10 or 15 mA flowing through it. That leads to a power dissipation of about 100mW. In this case that's okay since you're working with a relatively small current. Normally though you would use a PNP transistor and turn it completely on (saturation) to keep it from getting hot. PNP also prevents the problem that your base voltage has to stay above the emitter voltage, which varies based on the load. Again, though, it shouldn't be a problem in this application.
Make sure you put a resistor of around 1k in series with the base and the microcontroller to prevent large currents from flowing.
Watch out at the bottom because if all seven sements are on, the transistor will have a total of about 100mA, which is its maximum rating. You might want to first switch on half the segments and then the other half in order to keep the total current down. Of course this rating is mainly due to thermal effects, so it might be alright since you are multiplexing.
Regards,
Chris