I have seen in many tutorial videos people zoom in and out on Xcode to show the code a little better. I tried with 'control' + 'two finger swipe on trackpad' but this does not work on Xcode. What is the shortcut for zooming on Xcode?
Screen Zoom is a system-wide feature. In Lion, this is part of Universal Access.
The short version is:
Visit System Preferences->Universal Access->Seeing
And set "Zoom" to on.
You'll then be able to scroll using the control key and the scroll gesture of your input device.
Follow the instructions here for more detailed instructions.