I have a set that look very similar, bought from Amazon couple of years ago, quite substantial, only problem I had was that they would AF on my 40D but not on my 5D, otherwise good value for money
With extension tubes, the maximum aperture of the lens drops. When you get down to 1:1 magnification, the maximum aperture will have reduced by two stops, so your f/2.8 lens is now f/5.6, and an f/4 lens becomes f/8. If this reduction take the maximum aperture over f/5.6, most AF systems (not contrast detect AF, ie live view, that should work at any aperture) will shut down - assuming that change is correctly reported to the camera. Even if the change is not reported and the lens still attempts to AF, performance will be poor or it may just shunt around and not focus on anything.
You don't need AF when doing macro shots.
This is true, certainly when you get very close when normal AF, even when it's working okay, is often just a PITA.
The technique is to focus up manually to around the right distance, then fine tune by moving your whole body backwards/forwards very gently. Because depth of field is miniscule, you will see the plane of sharp focus changing across the subject as you move.
Camera shake is magnified at macro distances, and the effectiveness of image stabilisation reduced, so you need to keep shutter speeds up. But given the need for high f/numbers to get sufficient depth of field, everything starts to go against you unless in good, bright daylight. This is why ring-flash etc is popular.
