Undefined symbols: "_OBJC_CLASS_$ error
In general, this will occur when the code for BoxView is not being compiled into your target correctly.
You need to ensure that the target you're building has its corresponding box checked for your BoxView.m implementation file. Your question suggests that you've tried this, but here's a screenshot (from Xcode 4) just for clarity's sake.
A 'Clean and Build' never hurts, either.
I just want to add that Ben Mosher's answer is totally right. But there's another way to include the files to build in the Target settings.
Added Scenario
If your project has module dependencies(framework), rebuild them before building your main project.