Weird, I copied your code and it worked just fine.
You may try to replace MainActivity.this with this, as classname.this is only needed when working with nested class. And for your information: You can chain the toast call like this:
Toast.makeText(MainActivity.this, "I'm a TOAST and i not appear.", Toast.LENGTH_LONG).setGravity(Gravity.CENTER_VERTICAL|Gravity.CENTER_HORIZONTAL, 0, 0).show();
and methods normally start with a lower case letter.