These extension tubes do NOT have any electrical contacts to send lens information to your camera, therefore, the camera assumes there is no lens attached and shows f00. These extension tubes just increase the distance between your camera sensor and the lens which enables you to focus closer and achieve greater magnification. Needless to say, you lose any control over your lens while using these extension tubes the same way you do in reverse-lens macro technique.

However, there are some extension tubes available which has electrical contacts and allow you to communicate with your lens while being used i.e change aperture, auto-focus but they are way more expensive.

Ideally, you should set the aperture prior to detaching the lens off the body (by pressing and holding the DOF preview button while taking the lens off) and mount the extension tubes in between. Lack of AF shouldn't bother you much as the DOF is too shallow, you'd have to focus manually in most of the case.