Currently access to VO resources cover catalogues using so called cone search (CS) and images using simple image access protocol (SIAP). Toolboxes for accessing these are found in the "Data-Servers" menu.
The two toolboxes offer differing approaches to accessing resources, but fundamentally work the same way: locate what services are available by querying a registry and then querying individual services for a catalogue or image that covers a given region of sky.
VO resources are registered in a registry, so typically you find what services are available for a service type by initially querying the registry. To avoid this overhead GAIA has a cached version of a full registry response, but typically you'll want to refine these lists using additional constraints.
In the case of images, for which there are fewer servers and generally you'll only want a single image at a time, this step needs to be accessed using "View image servers..." dialog in the toolbox.
For catalogues, of which they are many, restricting the list of services is a more immediate need, so this forms the first part of the dialog.
See the individual toolbox help pages for more.