The DC/DC converter I have on my motorcycle is really basic. I got mine on eBay a while back for $10. (Finally beat Forkenswift prices on something!)
Typically, you can hook some sort of potentiometer up to those "trim" connections. Then you can use it to adjust the voltage up or down.
I am currently running the cycle WITHOUT an accessory battery, just to save space and keep things simple. The electrical load is one headlight, taillight, and turn signals - very few watts.
It has a "power switch" pin that was marked "Rem" and remotely controls power to the DC/DC converter. I trial-and-errored to fingure out which pin I had to connect that to and then wired up a basic 12v switch on the dashboard to turn the converter on and off.
As Tom said, you need to adjust your output to keep you accessory battery happy. I will have to do that if/when I ever add an accessory battery. It would probably be a good safety feature so that I still have horn and brakelights if something ever happens to my main pack.
The red arrow on the DC/DC converter is a not to myself for which pins to connect to power up the converter.
The left switch turns on the converter. Then I turn on the lights, followed by power to the controller, which is on the right handlebar in place of the original start/kill switch. Then I drive off!