Use the "+" and the "-" buttons on the left to add and remove libraries. Select the libraries you want to use and add albums to it by clicking on the "+" beneath the right preview pane to add libraries. You will be prompted to browse for an XML file you've exported from iTunes and after a while the albums will appear in the window.

When using the "UI View" you can select albums and change their sort key or description, or delete them from the library. Use "Peephole View" to see a preview of one style and "List View" to see a preview of another.

Use the export button to export an HTML version of the current library with the current style to a directory.

The results are, depending on the stylesheet, visible here or here.