Android : Actionbarsherlock call Fragmentactivity tab from activity

I have MainActivty and FragmentMain and FragmanettabA, FragmenttabB, How can i call FragmanettabA from my MainActivty? Itry this not work. But i call main FragmentMain from MainActivity working fine.

  button = (TextView )findViewById(R.id.download);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent intent1 = new Intent(MainActivty.this, FragmanettabA.class);
                startActivity(intent1);
            }
        });