Take a look at "ang_add" and "ang_rotate" in the manual.
There is an example code, which I'll just copy and paste here:
Code:

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// A camera is hunting a target and turns with them, keeping the target always in the middle of the view.
var cam_dist[3] = -500,100,100; // xyz distance vector from camera towards the target 	

function chase_camera(target)
{
  my = target; // The target Entity
  // calculate the camera view's direction angle to the target
  var cam_ang[3]; // direction angle of the camera to the target.
  vec_diff(temp,nullvector,cam_dist);
  vec_to_angle(cam_ang,temp);
  cam_ang.roll = 0; // roll didn't change vec_to_angle

  // permanent updating of the camera's position and angle
  while (1)
  {
    // place the camera at the right position to the target
    vec_set(camera.x,cam_dist);
    vec_rotate(camera.x,my.pan);
    vec_add(camera.x,my.x);
    // Set the camera angles to the camera view direction 
    vec_set(camera.pan,cam_ang);
    // and rotate it about the target angle 
    ang_add(camera.pan,my.pan);
    wait(1);
  }
}

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