Whoever made the QMC2 frontend for MAME is not extremely good at UI, sorry to say. Just saying. There isn't a strong distinction between certain parts of the UI, which can make things confusing for users. E.g., the machine-specific tabbed sections looks coextensive with the log which gives feedback on key processes a its bottom.
We are a cute and loving international community Ｏ(≧▽≦)Ｏ !