Each Pie menu can be accesed by pressing
it's shortcut and the Middle Mouse
You can change the keys for the Pie Menus by Right clicking on the Tray Icon Menu and chose Edit Pie Menus.
Don't worry if the shortcuts for the Pie Menius are the same as the ones in S.Painter, they will still work in S.Painter but on the release of the key. You can customize each Pie menu by adding buttons like in the Shortcuts window