Hi there! I do remember there being an issue with focus the first time I used them. It was because I automatically got real close to the flower. It was because I was TOO close (I was using the kit lens).
Remember that the tubes compensate for distance. Go to a flower, then focus, if you cannot keep taking steps back until you can. You will reach a point where they focus beautifully. I think I read this either in the extension tubes pamphlet or on some other site that addressed the use of extension tubes (maybe even on the extension tube manufacturer web site). Please let me know how it goes.
Remember that even with the kit lens the closest you can get to a flower and focus is about 11 inches or so. So just move your body until it focuses.