Please re-read carefully the steps 4 and 5 of the post. You must write your code to implement the message you want. In your case, as you want to send information to the Pixhawk, you must first pack the message and then send it.
The functions needed are detailed in step 4 for the mav_request_data, you must adapt it to the mavlink_msg_statustext. First use:
static inline uint16_t mavlink_msg_statustext_pack(uint8_t system_id, uint8_t component_id, mavlink_message_t* msg, uint8_t severity, const char *text)
static inline void mavlink_msg_statustext_send_buf(mavlink_message_t *msgbuf, mavlink_channel_t chan, uint8_t severity, const char *text)
You can see in step 4 how to define the variables for the buffer to finaly use
Serial.write(buf, len) to start the actual comms.
Hope it is clearer now.