What isn't 1 John 5:7 in modern translations of the Bible? The simplest answer is "because it's not in the original Greek manuscripts." I've blogged about the Comma Johanneum before. I've also blogged about why the NIV allegedly "omits" verses.
Here are several more helpful resources that I'll link to:
Dan Wallace, Greek scholar, textual critic and manuscript expert talks about the Comma Johanneum. Here's a post on the KJV in contrast to modern translations. It also contains video.
Here he address the "Conspiracy" behind the new Bible translations.