an ad hominem attack is a fallacy in logical argument. It involves making a case against the person or their background, not the reasoning or evidence they present in their argument. It's like saying that you can't believe Nelson Mandela's autobiography because he was once deemed a criminal and spent time in jail.
Credibility is a factor when one questions the accuracy of specific factual claims made (based on prior experience of getting facts wrong). I do think that it's not unreasonable to have some skepticism about even so eminent a paper as the NYT given its modern history and occasional severe problems with journalistic integrity. That said, I still read it (with a grain of salt, perhaps two).
roywhite seems to be concerned with using the NYT as a source; if he read the piece, he'd know that the story mostly summarizes what is in the grand jury presentment. If one is concerned about whether the NYT accurately summarized the GJP, a good solution to is to go to the source directly. [PDF]