Oh, right. Tiberium is tricky stuff - it hurts infantry that walk through it, making it a tactical element, and has to be able to grow, meaning that you can’t just plonk it down anywhere. You’ll find the magic alien crystal in the Place Object window, under Neutral/Misc_Natural. Tiberium comes from Tiberium Fields, which only appear as a lump on the map screen, but quickly blossom in-game. You can also place crystals by hand.