Welcome to 'Rangville! I've been using a Boomerang+ for a year now, and maybe I can give you some pointers.
The Trim knob is simply a gain control that prevents the 'Rangs input from clipping and causing distortion. First you set the 3-way switch to select the range that the knob will work in. You'll have to experiment with the switch. "Mic" is very sensitive (that is, it will cause the "Clip" LED to turn on very easily), "Inst" is less sensitive, and "Line" is least sensitive. You should adjust the trim knob just below the point where the "Clipping" LED flickers.
On my unit, the middle switch setting is useless, because it causes a tremendous amount of hiss. This must be a defect. The other two switch positions work fine. I found that for best playback quality, set the trim knob to well below the setting where the LED flickers. You will get clean sound even as you start stacking. The trim knob works for ALL loops, not just the first. If the 3-way switch is set correctly, the trim knob will be set relatively close to the middle of its rotational travel.
As far as volume levels go, it all depends on your preference. You'll want to experiment to get the best mix. It becomes very easy after a short while.
Hope I haven't confused you!