Michael, I presume you want to determine which icons show in the Notification
Area (next to the clock on the Taskbar) and for how long; this is what the
"Customize" button leads you to.
To make this button "active," you first need to check the option "Hide
inactive icons" which is to the left of the Customize button. As soon as you
check this, you will see Customize activate so you can click it.
Paul :^)