Lo is required if the direct object is not a noun. "todo" is used as an adjective or a pronoun in Spanish and cannot be a direct object by itself.
I translated it as "You tell it all to me." which is a more direct translation, but does mean the same as "You tell me everything." which is more commonly said in English. Of course, DuoLingo did not accept it, but I reported it as also correct though less common.
That would be "Me lo dices todo."
"lo" is the direct object pronoun which replaces the noun when it is not in the sentence. Usually, todo is by itself used in the nominative case (subject, predicate nominative) or as the object of a preposition, but usually "lo" is used with it when it is the direct object. "lo sabes todo." , "se lo comió todo", etc. http://dictionnaire.reverso.net/espagnol-anglais/dices http://dictionnaire.reverso.net/espagnol-anglais/todo http://www.studyspanish.com/lessons/iodopro.htm
Maybe a native Spanish speaker could step in and let us know if lo is dropped in casual talk? I know "lo" is required even for a noun if the direct object is at the beginning of the sentence. http://spanish.about.com/od/usingparticularverbs/a/gustar_a.htm
We have a similar word in English. "Recount" means "to relate in detail". He recounted the story. He told the story. Yet, "recount" can also mean "count again". The indirect object is "to me" and "you count it to me" makes no sense, but "you tell it to me" or "you tell me it" works and gives us the correct meaning to use. You have the correct translation for "Tú lo cuentas para mí." You also ignored the word "todos" which means all.