If you carefully go through Ardupilot code, you will find all the mavlink communication related function are put in file GCS_Mavlink.pde. If you wish to receive particular message you will have to look into and modify this file.
Mavlink message attitude(#30) is handled in this function within ardupliot. So you will have to modify and the ardupilot code and call this function at desired time interval. Similarly you can find relevant functions for desired messages already build within code.
If you wish to send any mavlink message on your own then I would suggest you to go through particular mavlink message file. All transfer functions related to particular message are defined within message file. For example, in order to send attitude(#30), go to mavlink_msg_attitude.h and refer to function mavlink_msg_attitude_send().
One should also refer here for more mavlink related information.