It seems that if jury nullification is a right, then jurors should be told about it.
In general they are not told. In fact, I am not aware of any jurisdiction where they are told by the judge officially. In fact judges will normally charge a jury that they must accept the law as stated by the judge, and ignore any other source of the law, whether they like it or not. But the Judge has no way to enforce such a charge.
According to the Wikipedia article
The 1895 decision in Sparf v. United States, written by Justice John Marshall Harlan held that a trial judge has no responsibility to inform the jury of the right to nullify laws. It was a 5–4 decision. This decision, often cited, has led to a common practice by United States judges to penalize anyone who attempts to present legal argument to jurors and to declare a mistrial if such argument has been presented to them. In some states, jurors are likely to be struck from the panel during voir dire if they will not agree to accept as correct the rulings and instructions of the law as provided by the judge.
A 1969 Fourth Circuit decision, U.S. v. Moylan, affirmed the power of jury nullification, but also upheld the power of the court to refuse to permit an instruction to the jury to this effect.
We recognize, as appellants urge, the undisputed power of the jury to acquit, even if its verdict is contrary to the law as given by the judge, and contrary to the evidence. This is a power that must exist as long as we adhere to the general verdict in criminal cases, for the courts cannot search the minds of the jurors to find the basis upon which they judge. If the jury feels that the law under which the defendant is accused, is unjust, or that exigent circumstances justified the actions of the accused, or for any reason which appeals to their logic or passion, the jury has the power to acquit, and the courts must abide by that decision.
Nevertheless, in upholding the refusal to permit the jury to be so instructed, the Court held that:
…by clearly stating to the jury that they may disregard the law, telling them that they may decide according to their prejudices or consciences (for there is no check to ensure that the judgment is based upon conscience rather than prejudice), we would indeed be negating the rule of law in favor of the rule of lawlessness. This should not be allowed.
It is not so much that jury nullification is a right of the jury, as that there is very little right for the prosecutor or judge to inquire into why the jury acted however it did. If there is a suspicion that the jury was bribed, or influenced by prohibited communications, that can be looked into. But otherwise a jury is like an oracle, its actions have no specified reason or justification, they are whatever they are.
The judge (or an appeals court) can set aside a jury verdict on the grounds that no rational jury could find in a particular way -- this is mostly used to overturn convictions based on insufficient evidence. But a jury has almost total freedom to believe of disbelieve any witnesses, so if it disbelieves, it could acquit, regardless of whether it rejects the law under which charges are brought. So there is no way to tell if a particular verdict was based on nullification, or on disbelief of the witnesses, or some other possible ground.
In any case, there is no provision -- that I k now of -- to set aside a jury verdict on the grounds that it was an instance of nullification, so inquiring into whether it was would be of little point.
This attitude toward jury verdicts goes back to the very early origins of trial by jury, when it was a replacement for Trial by Ordeal. The Ordeal had been considered a way of asking God to decide the issue, and there was no way to ask God to clarify the decision. When it was replaced by jury trial, no way to ask for clarification was considered possible there either -- the jury was said to voice the decision of the community at large: the formal term for jury trial was "to be tried by the country". See C. Rembar's The Law of the Land and H.C. Lea's The Duel and the oath for more on this history.
This article reports on recent cases where juries have refused to convict in Marijuana cases.