Guess I assumed you knew more then you actual do ....
As Pestilence already indicated, the Material Editor (ME) is only a "window" on your unlimited amount of textures present in your scene. You can have thousands of materials, but you can only view 24 of them at once.
To free up a (ME) slot, you hit the X (reset map/mtl to default) at the button bar under it.
You'll be asked to delete only the material in the ME or in the scene also.
Choose (ME) only, as we want to keep the material in the scene, no?
(MB) = first button "get material", choose option selection, so the material of your selected object will appear in your MB. You can then drag/drop a material from your Material Browser (MB) into a (ME) slot.
You need to read the help and tutorial files more carefully
