Struggled with it also myself. That of course until a friend told me to move upwards with W-S-A-D buttons, while at Skill menu tab.
Simple, but confusing for new players as there isn´t any indication.
As Saku_Ice mentioned you can use WASD. You can also move upwards using the mouse cursor, I think you need to keep RMB or LMB pressed but cant remember.Also try the scroll wheel.