To debug an Xcode project, we first need the path of the debug build, which we can get by right-clicking on the Products folder in the Project Navigator, and selecting Show in Finder:
Now add that path as the argument to the lldb command (dragging the build from the Finder into the Terminal window is a pretty quick way to do that):
~ lldb /Users/.../Build/Products/Debug/5623_18_code.app
Hit return. This will produce something similar to the following output:
(lldb) target create"/Users/.../Build/Products/Debug/5623_18_code.app"Current executable set to '/Users/.../Build/Products/Debug/5623_18_code.app' ...