customise tools doesn't work

I tried to customise the drawing view by adding some buttons. I right clicked and chose customise then I chose the onion skin buttons clicked the arrow to move them from available tools to toolbar then clicked ok but I can’t see them. Where are they?? I have tried this several times but can’t seem to add any buttons. Am I doing something wrong? I don’t get how something seemingly so simple could be so confusing.

Which software and version are you using?