It's hard to say whether the article is required, since in Latin they're assumed and not explicitly expressed.
A quick swing through relevant web loci shows that usage with articles is fairly common. It looks like it could go either way.
Mens rea, or "guilty" intent, deals with what the defendant needs to have been thinking at the time he or she committed the actus reus for criminal liability to attach. In order to be guilty of most crimes, the defendant must have had the mens rea required for the crime he was committing at the time he committed the criminal act. As with the actus reus, there is no single mens rea that is required for all crimes. Rather, it will be different for each specific crime.