No one told Nixon anything. Nixon assumed his good friend Att'y Gen'l John Mitchell was behind the breakin. Mitchell assumed it was Nixon. Neither one asked the other directly. Nixon had the incredible bad luck of assigning John Dean to find out who was behind the breakin. And Dean, being the culprit himself, used his position to prevent Nixon from learning the truth. This is in Silent Coup, which is definitely worth reading.
Silent Coup was awesome. John Dean also has proven his "worthiness" for higher things by his whiny behavior and turn-coat commentaries. Whenever I see him "on", my tv goes "off".