Many tasks ask you to pick stimuli to use. For example, here's the Simple Schedules task:
It would like to use two stimuli (the response object and the marker object). At the moment, the stimuli "SS_bluebox" and "SS_redbox" are being used. But you could change that. If you know the name of one of your stimuli in the visual object library, or the name of a predefined stimulus, you can type that name directly. Otherwise, press the Set button next to the object. You'll get a choice like this:
This is the list of all the objects currently defined in your Visual Object Library. If there aren't any, or there aren't enough, you need to edit the visual object library to add some more. If you see one that you want, highlight it (by clicking on it) and then click OK. If you want to use a predefined object, click Choose predefined object instead, and you'll see this:
Choose a stimulus set from the predefined stimuli available; then choose an object within that set, and press OK.