Intuos includes various software offerings. More details are available here.
The Nibs are stored inside the tablet. To access the Nibs, simply slide off the removable panel on the back of the tablet, the nibs are stored in the middle of the tablet next to the Nib extractor.
Be sure you are quickly tapping the tablet twice in the same place on the tablet active area. Wacom recommends using the side switch to double-click. You can also open the Wacom Tablet Properties and navigate to the Pen tab. Try increasing the Tip Double Click Distance, or using a lighter Tip Feel.