ok, i fixed it. when i updated this project for 3.0RC0 (from an earlier version), i did not copy over the invisible ".cocos-project.json" file.
i simply made a new file named .cocos-project.json in the root of my project's directory with the following contents:
i have a question, though: why did the cocos2d-x developers decide to make this file an invisible dot-file? there are plenty of other config files that aren't invisible. what is special about this one?