Good evening, I would be happy to assist you with this.
The fact that you have disclosed your arrest is good as it demonstrates your honesty. Now they are requiring your criminal record check. I would assume that you have already provided this to them in the course of your application, however if you have not, then you can apply for same from your local police service.
Any police or court records held locally regarding this arrest would have been purged long ago. Because the US does not recognize Canadian Pardons, they are probably looking to see whether this has been cleared as a result of the granting of a Pardon.
Your official record, as provided by the police will show them what convictions or criminal involvement you have had if any. If you were not convicted, then this will not be on your record and will corroborate the fact that you were arrested and not convicted of anything. You may also be able to obtain a confirming cover letter to accompany your record, from your local police, confirming that this is your record in its entirety and nothing else exists with regard to your arrest or any other involvement.
There are many people who have been arrested but not charged, or arrested but not convicted. All the US usually requires is your official record, or notice confirming that none exists. If there is no record, then a letter confirming same may be of use. I would suggest showing the police the request from the US to see if they will assist in clarifying.
Please let me know if you require anything further as I am happy to continue discussing this.