This issue came up in a dissertation I was editing today. If you are quoting a source that does not have page numbers but does have chapter numbers, you can cite the chapter number. Note that APA does not abbreviate or lowercase the word Chapter in the citation (see section 6.19 of the APA manual). Here is an example of how to cite a chapter:
Note that if the sections have names instead of numbers, the word section would go after the section name:
Citing the paragraph is important to help the reader locate the quoted material (especially in long sections, but still include the paragraph number even in short sections; see APA 6.05). Also note that in such citations, the word paragraph is abbreviated to p, similar to how the word page is abbreviated to page is abbreviated to p.