I was able to run my app in the Simulator from Sencha Designer. However today I installed Xcode 4.3.2 and one of the things I noticed is that Xcode and the Simulator and so forth are now installed in /Applications rather than in /Developer. During the installation it prompted to move /Developer to the trash.
Now, in Sencha Designer it doesn't recognize that I have the iOS Simulator installed. I do not see any place to set the path to the Simulator.