On creating a new Android Application Project, MainActivity.java under its package name should be generated automatically in src folder &
Since this query isn't solved, I would like to add from my experience;
I agree with "SK17" 's advice.. The latest eclipse software somehow generates the MainActivity.xml file by choosing 'Empty Activity' instead of 'Blank Activity'.
Nevertheless, for some systems, you might also try changing the target SDK to Android 'JellyBean' or 'KitKat' rather than 'API21'.