Previewing an icon library is as simple as clicking on the "Libraries" tab at the bottom left of D3M. After that, all libraries can be previewed by clicking on the "eye" icon that appears up under your curser while hovering over each library.
After you have previewed a library, you can then chose to toggle on/off the library by clicking on the box that is to the left of the libraries name.
That's all there is to it! Check out some other help articles that go further into detail about using the icons within these libraries.