Use of Custom Message Structure for MessageQ package

Hi,

I'm trying to create an own MessageQ_Msg like structure. As mentioned in the Documentation I add the MessageQ_MsgHeader structure at the beginning

struct I2CTaskMsg
{
    MessageQ_MsgHeader    header;
    I2CTaskMsg_TRX        trx;
    UInt8                slave_addr;
    UInt8*                pTXData;
    UInt8*                pRXData;
    UInt8                numTXData;
    Uint8                numRXData;   
};

typedef struct I2CTaskMsg I2CTaskMsg_t;

The next thing I try to do is allocating the message. As mentioned in the documentation and in http://e2e.ti.com/support/embedded/f/355/t/106813.aspx as size argument I use the size of the structure which gives me the first compiler error. In the url above it is told that one should cast (I think to the MessageQ_Msg struct or vice versa to my own Message struct). Yet casting gives me compiler errors as well.

I2CTaskMsg_t        msg;

msg = (I2CTaskMsg_t)MessageQ_alloc(HEAPID, sizeof(I2CTaskMsg_t));

"../src/dummyTask.c", line 15: error: cast to type "I2CTaskMsg_t" is not allowed

if ((MessageQ_Msg)msg == NULL)
 {
        //error
}
// set own Queue as ReplyQueue for Message
MessageQ_setReplyQueue(dummyTask_MessageQ, (MessageQ_Msg)msg);

"../src/dummyTask.c", line 16: error: invalid type conversion
"../src/dummyTask.c", line 21: error: invalid type conversion

Any explanation what I'm doing wrong, or even better some source code telling me how to do this. since at least the user manual or cdoc/doxygen don't tell much about this.

Thanks

Stefan