when you unlock the plastic cover on the bar, underneath you will find a fixing plate held on by a torx bolt (you should have been given an allen key for this, if not you need a T40 Torx bit) remove this plate on both sides of the bar.
now all you need to do is peel back the rubber strip from one end on both sides of the car (it comes off with a twisting motion towards the outside of the car, try it at one end you will see it is actually easier than it sounds. It is also very easy to refit. kind of like a seal round a door) to where you want the bar to be.
Underneath the rubber strip you will find fixing brackets that match the lug on the roof bar (my mounts that i use ar roughly 2" in front over the front door and about the same over the back door). clip the fixing plate onto the bracket and refit the bolt on both sides. when you are happy with the position tighten the bolt and lock the plastic cover.
Repeat at the other end of the strip with the other bar.
Now all i did was twist/bend the rubber strip round the roofrack mounts and re-fit into the channel (so i could put them back to normal when i take the roofrack off).
this twisted strip has never moved and i and my family have traveled/camped our way around Europe this summer with a roof box mounted on the bars to carry our gear (which was great fun