A marijuana "ACD" (adjournment in contemplation of dismissal") means that the case will be adjourned for one year. If you are not re-arrested during that time period, the case will be dismissed and sealed pursuant to CPL 160.50 (a non-marijuana ACD consists of a six-month adjournment before dismissal).
In my experience, the fact that an individual received an ACD (and the accompanying arrest charge) sometimes shows up on rap sheets, despite the fact that the case has been sealed. Take heart, however: the mere appearance of the ACD on a background check does not mean that your case was not sealed. No one (except you and your designated agents) can access the court file for your case absent a court order, which is not easy to get.
While I cannot predict if the ACD will prevent you from getting the job, your receipt of an ACD means that your case was ultimately dismissed - which, in turn, means that you still, technically, have a clean record.
Finally, if the job application asked you if you were ever arrested for a violation (or crime, if you were charged with criminal possession), you need to answer honestly. More than likely, while the ACD itself won't interfere with your getting the job, a falsehood on the job application will.
If you think it is an issue, I suggest obtaining a certificate of disposition from the criminal court clerk. This document is very easy to get and serves as an official confirmation that the case against you was dismissed.
From the NY Bar Association:
"Any arrest which is not pending or did not result in a conviction is not considered an arrest according to New York criminal laws. N.Y. Executive Law Section 296, subdivision 16, prohibits such a question with limited exceptions, for example on applications to become a policeman. A file that has been sealed also allows the applicant to answer no. New York Criminal Procedure Law Section 160.50 which requires the sealing states that when a criminal action terminates in a person's favor all records shall be sealed "and not made available to any person. An ACD is a form of dismissal of charges and is not a conviction."
I concur with what was previously posted with one exception. Though I agree that one must always be honest, if your case is already dismissed and sealed as a result of a MACD then you may legitimately answer that you were never arrested. I am providing the relevant sections of CPL 170.56 for you. Specifically note subparagraph number 4 where it states that the arrest is a NULLITY.
(3). Upon or after dismissal of such charges against a defendant not previously convicted of a crime, the court shall order that all official records and papers, relating to the defendant's arrest and prosecution, whether on file with the court, a police agency, or the New York state division of criminal justice services, be sealed and, except as otherwise provided in paragraph (d) of subdivision one of section 160.50 of this chapter, not made available to any person or public or private agency; except, such records shall be made available under order of a court for the purpose of determining whether, in subsequent proceedings, such person qualifies under this section for a dismissal or adjournment in contemplation of dismissal of the accusatory instrument.
(4). Upon the granting of an order pursuant to subdivision three, the arrest and prosecution shall be deemed a nullity and the defendant shall be restored, in contemplation of law, to the status he occupied before his arrest and prosecution.