If a Digital Object Identifier (DOI)is listed on either a print or an electronic source it is included in the reference. A DOI is a unique alphanumeric string that is used to identify a certain source (typically journal articles).
The DOI is often found on the first page of an article. For more information on DOIs and sample pictures indicating where to locate a DOI on a source, check out pages 188 to 192 of the APA Manual and/or this helpful DOI flow chart put together by the people at APA.
Curious about how to find a DOI?