How do I do this?

If you are on the prolog isle then yeah, I think that's still an issue. The best thing you can do is finish the prolog isle with the set up they give you and then it should stick you on the Isle of Dawn which is the starting island where you can then move you're skills around. Otherwise just look for a symbol thing on you're action bar and click it and it should let you freely move them around if you are not on the prolog isle part.

It can also bug out sometimes if the bar is near the edges of your screen. Just drag the bar away from the edge. Also, make sure your skillbar is not locked.

I had the problem where my skill bar was locked, so I couldn't drag/drop anything to it.

To unlock it, you have to click on one of the red/yellow buttons that appears near it.

Right hand side of the bars...top row...is the lock/unlock option. :)