MIC Calculate?

Section 4.4 of the LoRaWAN specification is quite clear about MIC calculation. Here's a simplified version of some code I wrote a couple of months ago:

// msg = MHDR | FHDR | FPORT | FRMPayload
var msgbuf bytes.Buffer
msgbuf.WriteByte(mhdr)
msgbuf.Write(fhdr)
msgbuf.WriteByte(fport)
msgbuf.Write(payload)
msg := msgbuf.Bytes()

// B0 =  0x49 | 4x 0x00 | Dir (uplink=0x00/downlink=0x01) | DevAddr | FCnt (4 bytes!) | 0x00 | len(msg)
var blocksbuf bytes.Buffer
blocksbuf.Write([]byte{0x49, 0x0, 0x0, 0x0, 0x0})
switch mType {
case UnconfirmedDataUp, ConfirmedDataUp:
	blocksbuf.WriteByte(0x00)
case UnconfirmedDataDown, ConfirmedDataDown:
	blocksbuf.WriteByte(0x01)
default:
    // nonononono
}
binary.Write(blocksbuf, binary.LittleEndian, dataPayload.FHDR.DevAddr)
binary.Write(blocksbuf, binary.LittleEndian, uint32(dataPayload.FHDR.FCnt))
blocksbuf.WriteByte(0x00)
blocksbuf.WriteByte(byte(len(msg)))

// Append msg to B0
blocksbuf.Write(msg)

blocks := blocksbuf.Bytes()

hash, _ := cmac.New(nwkSKey)
hash.Write(blocks)

return hash.Sum([]byte{})[0:4]

You could also have a look at how it's done in the github.com/brocaar/lorawan package.