In order to communicate with a tag directly on Android you have to obtain a reference to the detected tag from one of the new NFC intents (NDEF_DISCOVERED, TECH_DISCOVERED, TAG_DISCOVERED). Setup an activity to receive these events and then extract the Tag object;
Tag tag = getIntent().getParcelableExtra(NfcAdapter.EXTRA_TAG);
MifareClassic mifareClassicTag = MifareClassic .get(tag);
mifareClassicTag.connect();
// The transceive command sends command directly to the tag. Internally it wraps the given command in a direct transmit command and sends it to the NFC chip which forwards it to the tag
mifareClassicTag.transceive(...);
See the developer docs for more info for NFC on android http://developer.android.com/guide/topics/nfc/index.html