I'm using Mendeley, and trying to cite this article using ACM citation style:
American Diabetes Association
Should the author be "N/A" or the name of the association?
In this case the author is not unknown. The author is the association. In this case, "American Diabetes Association".
Even though there are no direct guidelines but this is the case with almost all citation styles. Also, if you export this citation, the name of the author reads as "American Diabetes Association". Zotero also reads the name as that. The same holds true if you look up the article on PubMed. Thus, I think the name should be the name of the association.
You need to mention the name of the author in ACM citation style. Please refer this link to know more about the ACM citation style http://www.cs.ucy.ac.cy/~chryssis/specs/ACM-refguide.pdf