I am having difficulty locating articles listed in the required supplemental readings for my class assignments
Unfortunately, not all supplemental readings will be available in the Online Library, but I can show you the way to find those that are. If you are unable to find your required additional readings, you should contact your professor (there may be some in your Document Sharing area of an online or blended course, or potentially something that you get from the professor or the library [I'm in AU Twin Cities, so I don't know how your library works]).
When you launch the Online Library, click on the "Find Articles" section to get to a list of the article databases. In the alphabetic list, the first link is A-Z of Online Journal Titles - which isn't a regular database but a listing of where the available journals may be found full text. This database is searched by using the Journal title from your citation. For example, if you were looking for an article from The Journal of Psychoactive Drugs you would type that in the search box and either hit the "Enter" key or click on the red "Go" button. Underneath the search box it takes you to the closest title to what you typed in (if it is not available, it will automatically take you to the next title alphabetically), and underneath that title you can see a list of databases that have that title full text, and under each database it will list which years are available. If we wanted something from 2009, we could choose any of the listed databases, if we wanted 2001 we couldn't choose the first listed one because it starts in 2005, and if we wanted 1997 we could only choose the last listed database because it starts in 1997. If we wanted anything before 1997, we are unfortunately out of luck. You would click on the database name that has your year covered and then you have the option of searching within that title or navigating through the year, volume and issue and looking at all articles available to find the one you need.
If you need to find books, there are multiple book databases to search, and you can do them individually by entering them via the Find Books page. ebrary is the largest of the regular book databases, with great coverage of a broad range of subjects, NetLibrary is smaller, and tends to have more of a business/education slant to it, and PsycBOOKS is, as it appears, psychology related titles (published by the APA).
I hope this helps you on your way!
AUTC Library (answering for AU San Fran Library)