Steve, according to the manual, you need to have the throttle down all the way, AND the throttle trim down all the way, then plug in the pack. You should get a series of tones and continuous LED lamp.

Since you got everything else working I'm guessing you have the esc plugged into the correct channel on the rx for your system.


The Horizon esc's can be finicky at times. On my E-Flite airliner, I have to have the throttle up fully, plug in the pack, listen for tones, then move the throttle to off, to get them to arm.
I ended up adding some down throttle trim too as once in a while it seemed like the esc's didn't recognize throttle off. Now it works, weird, but it works.