When you are at the spot where you have to proceed to El Capacito, tell your 
men to go
to the big pointy statue in the center and take cover. Then, proceed to 
shoot down the
helicopter first before continuing. A few RPGs will be required before it 
will go down.
If you run out of RPGs, go back to the previous spot where you had to shoot 
the APC
vehicle to get more supplies.