OCUnit, NSLog, and XCode 3.1
Just learning this stuff myself and had to poke around to answer that very question (XCode 3.1.2).
Go to the Build Results window after running. It may not open automatically, depending on configuration. Build / Build Results menu option will open it if needed.
In the middle area between the two panes, you'll see 4 buttons on the left. The 3rd icon is just 3-4 horizontal lines. Click that to toggle the build transcript. That shows all build steps and NSLog() output from the run.