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I have a DMA2100, and I don't recall having to install anything from its' setup pack. The usual hiccup with these things is that sometimes updates can mess with the extender info in Media Center.
A possible fix would be to start the DMA2100 and go to the setup screen and find the 8 digit code for the extender. Then go to your PC and open MC. Go to setup extender and make sure that the 8 digit code is the same. If it isn't, enter the code that you saw on the TV into MC.
Give that a try and post back with the results. Also, please fill out your system specs so that we know what kind of hardware you have. It makes it easier to help out.