rkt3ct:
If the material was part of the original installation (in other words, not one you created or imported), you can do a
New & Reset, which will open a blank file with the original materials. You can assign this material to an element (like a Slab) and copy then paste it into your current project, the material will be added to your project file.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David