This is assuming you have run the laser wizard and not using your own custom built pipelines.
Adding materials to the Laser wizard set of pipelines is easy.
Just go into Tables > Laser materials.
You will be presented with three sample materials that the wizard has automatically created.
Deleting a material
Click the X button next to the row-key cell, and the material will be deleted.
Adding a material
Click the + button in the last table row, and an empty row will be added. You can now add info to every cell, by clicking the pen icon in each cell.
If you are wondering about how to figure out speeds in mm/s, please see this article.