After installing and setting up the addon you can find the bsp importer under: File -> Import -> ID3 BSP (.bsp)

There are several options to change the import.

Subdivisions will tell the import how often patch meshes will be subdivided. This can't be changed after import.

The minumum lightmap atlas size can be used to grow the atlas size. If the needed atlas size exceeds this value, the needed atlas size will be used.

Vertex lit unwrap will change the lightmap uvs for all vertex lit surfaces. Primitive packing is prefered for baking lighting. If you plan to change vertex lit surface to lightmapped surfaces, use Diffuse UV Copy and pack the vertex lit surfaces afterwards.

The import presets are build for different tasks:

Preview: Will import the map with the lighting that is stored in the bsp file. This is just for previewing purposes.

Entity Editing: Will import the map like the preview preset, but will also import all bsp entities. This enables entity patching.

Rendering: This will import the map without precalculated lighting. This will try to import light sources. Use this preset for baking lighting.

Brushes: This will import the brushes of the map. No particular usecase except for checking how things were made.

Shadow Brushes: This will import brushes with non transparent shaders. You can use this to minimize shadow leaking when baking lighting.