A genetic fallacy occurs when someone dismisses information or argumentation because of its source - in this case, a senator wants to dismiss the claims made by "the media" simply because those claims were made by "the media," and not on any grounds having to do with the truth of the claims themselves. It may be true that the media is generally unreliable, and it may be true that the allegations are false, but it is fallacious to assume that the latter follows from the former.
EDIT (in response to comments): Try not to read too much into this specific example. Of course the senator is within his rights to point out that the media tend to be unreliable, and that this makes it less likely that statements coming from the media will be true, but these are superfluous details that do not enter into the question of whether a genetic fallacy has been committed. And insofar as the senator's denial of the accusations consisted only in his assertion that they came from an unreliable source, they are fallacious.
Perhaps I can modify the example a bit to elucidate the issue...
Having been accused of corruption and taking bribes on the Seattle Nightly News, and asked whether the accusations had any truth to them, the senator answered that they could not possibly be anything but lies: "We should not believe the things we hear in the media," he said, "because we all know how very unreliable the media can be."
So the senator may or may not be right in remarking that journalists are liars, but he is certainly wrong in asserting that that fact makes the allegations false.