Dumb question, but did you add the web.h and web.cpp files to your Xcode project? The project won't see files unless you add the files to the Classes folder in the project.
And, very important, make sure that the 'File Type' dropdown is set to 'Objective-C++ Source' (in the far right column when you select a file) for the files you add to the project. This way it treats the files correctly and uses the right paths and stuff. I was very frustrated with the same errors and finally found that solution.
And a caveat, you can use agk::Sync() to update a display, but don't use it in a loop that is waiting for any events like GetPointerPressed. In Tier 2 those processes are checked after each call to the agk::Loop() method by the main program. This is one of the only flaws (in my mind) about Tier 2. It means setting up state engines whenever you need to do something that waits on some sort of user input.
Cheers,
Ancient Lady
AGK Community Tester