The Accessories Catalog is accessible in both Physics and Discrete Events Mode.
Its purpose is to allow users to import custom items for use within models, but with only very basic properties as opposed to coding a custom catalog item for example.
To import an item, right click within the catalog and select Import.
The supported file formats for import are shown in the drop box in the bottom right of the import file dialog.
One thing to bear in mind when importing CAD files is that the dimension units will differ between it and Experior.
That is to say the CAD drawing will generally be measured in metres while Experior components are measured in millimetres.
This could lead to the item, when dragged into the model, being to large for the floor and needing re-sizing for it to be visible on the model.