As with Academic Search Complete, look for the Subject Terms index. In PsycINFO and PsycARTICLES, it's called the Thesaurus, and it's at the top of the page in the menu bar.
When you click "Thesaurus," you get a new box with the option to search for subject terms. Let's search for "memory."
This will get you a new box with links to "memory" and associated terms:
You can add terms to your search by clicking the box on the left of the subject term, then clicking "Add to search" at the bottom of the box.