Hello!
Please see your triggers attached. In the constants you can set the maximum liability of the back bets (the first bets you place on one or more selections), the divisor for the prices of lay bets and the ranks of the selections you want to bet on. If you want to bet only on one selection, say, the third favourite, then simply set rank1, rank2 and rank3 to 3.
If you import your selections from the file (i.e. you do not choose them by rank), then delete the first condition (Selection's Rank is in list) from the trigger called "backing".
Mcbee is right, you have quite low chances to have all lay bets matched. Although you can try reducing the divisor and playing around with ranks of the horses to back on.
Make sure you have tested the triggers in Test Mode.