Tab order for editable text fields and buttons is controlled entirely by the Flash player. There is currently no way to specify your own tab ordering.
The Flash player uses a formula to calculate the tab order of editable text fields and buttons that is essentially left to right and then top to bottom. The formula is:
k_x * x + k_y * y
where x and y are the x-coordinate and y-coordinate of the upper left corner of the text field or button (based on the upper left corner of the movie). If the fields or buttons are in a movie clip, the movie clip position is taken into account. The values of k_x and k_y are constant:
k_x = 1/3 k_y = 2
Note: This formula is calculated dynamically. That means that if you have text fields or buttons in a movie clip, and that movie clip is dragged, this can alter the tab ordering.