Fiddled with the demo for a little while until I built something capable of achieving orbit with enough fuel left for re-entry.
Maybe I'm just horridly inefficient, or the SRB's that come with the demo are crap, or both... but I couldn't reach orbit without 7 of them. 4 in the first stage, 3 in the second. After that the ME had just enough fuel to reach a stable orbit. It works consistently, but 7 SRB's seems really excessive to me.
I originally tried it with just one stage of 3 SRB's, but once they were spent, the ME didn't stand a chance at maintaining the upward momentum.