No visible @interface for
There are lots of reasons it could happen, but generally it's saying that at the line of code it flags, it doesn't see any evidence that the selector you are referencing is in the interface of the type it thinks the object has.
In this example,
They declared operandStack with the wrong type.
In this one
They had a typo in the selector name
Any chance you are on Xcode 4.2 (or less), running code that was written on Xcode 4.3? In 4.3+ Xcode doesn't require you to declare private methods at all, it just assumes that methods written in the implementation file (without declarations in the interface) are private. But in Xcode <= 4.2, this will throw an error and those methods need to be at least declared privately