To have a balloon showing when the mouse hovers over a Planner item, set Planner.Balloon.Enable = true. It is important that when you create the Planner programmatically or reparent the Planner control, that Planner.Balloon.Enable is only set to true AFTER the parent for the Planner control is set.

Also check if theming is enabled in the project options. Only when theming is enabled in the project options, the balloon will be visible.