I think the back lines on my 10m charger is a bit too long, they are a bit slack during flight and the kite feels a bit unresponsive.

I saw the tuning video from PL and it mentioned that the best thing to do is to try shorten the back line. But how exactly do I do this? the connection at the kite is a loop instead of the knots and don't allow for any adjustment..

Pigtails on the front lines. Don't the Charger's have a couple of knots?

What bar are you using? With the Navigator bar and stock lines the back lines shouldn't be too long. There are knots on the front leaders the kite to shorten the front lines but just the vpc adjustment on the rear of the kite. If anything I'm playing with pulling in the trim to shorten the front lines a little once in a while.
How is your trim set - all the way out and the kite might even be a bit stally if you fly in lighter winds.
I can't see needing rear extensions but that's what you would have to do unless you tie a knot in your rear line leaders coming of the bar and larks head onto lower knots.

I have the Navigator bar. Flying it today, I play with the trim set a bit and the back line now seem pretty good.
I know this is a completely noob question, but am I suppose to use the trim to adjust the power of the kite? Or is it mostly to adjust the angle of the attack to suit the wind condition?

Use bar trim to adjust power of kite

Use VP3 to adjust charactist of kite to your riding style.

Most leave everything on factory setting and just use the bar trim.

Frostbite - yes and no of course, pulling in on the trim strap/rope with "permanently" shorten the front lines and so alter the angle of attack and all that is associated with that - less power, less effect when sheeting in, kite more inclined to shoot to the edges of the window ... and you can trim the kite to find the sweet spot.

Keeping a good stance for me means the bar is close to the chicken loop and if the wind is near the top end for that kite, it means that I'm over-powered - pull some trim, and I can hold a good stance and ride comfortably. That is mostly what I use trim for ...

adjusting the trim has a lot to do with what happens when you pull the bar all the way in.
When you have the trim line pulled all the way in, the kite is max depowered in the sense the back lines are really slack with the bar out and take up the slack when the bar is pulled in. It will not give you much of a change in angle of attack. Then as you adjust the trim line the rear lines start to tighten so as you pull in the bar you get more of a dramatic change in the AOA - which also is part of getting more power.
The other part of power is relative to kite speed - if the kite is moving really fast (with the bar out) and you feel bar pressure then when you pill in the bar you'll get a big power increase.
How you set that trim amount varies from higher winds through to light wind - what you don't want to do is stall the kite or have it drop back when you pull in the bar. So start with the trim pulled in to launch (de-powered) and then use the bar to add tension to the rear lines and also slight adjustments on the trim to watch for any tip clapping or overflying if underinflated.
I usually then fly the kite around and adjust trim so that when I pull in the bar I get a good range to max power I want. If you are out riding and the wind ramps up or gets gusty you can then re-adjust the trim pulling in a little etc.
There are more subtle things that you can play with and get a feel for what happens and learn a bit more about how you like the trim setting in different conditions and uses. On water I want power and in a buggy or skiing fast I might ease of the power range a little to keep the speed up instead.