I'm not sure how to cite my source for a figure in my appendix (APA 6th edition). I'm also wondering if my caption should appear above or below.
The caption should appear directly below the figure, and should contain the information about the source in the following format (taken from APA Manual 6th Edition, p. 164):
[Figure number and description]...Adapted from "Central Perceptual Load Does Not Reduce Ipsilesional Flanker Interference in Parietal Extinction," by J. C. Snow and J. B. Mattingly, 2008, Neuropsychology, 22, p. 375. Copyright 2008 by the American Psychological Association.
Notice that the figure caption contains a complete citation, but in slightly different format than on the reference page. Also note the last statement--this is truly important only if your paper will be published, since it is a statement of permission to reuse the figure. Alternate versions of figure captions have "Reprinted with permission" instead of or in addition to a copyright statement.
See the APA Manual (6th ed.), pp. 150-167 for more on figures.