Hmmm, that is odd. The commands //sphere and //hsphere work fine for builders at least. I don't know the current permissions for anything other though. Try again but make sure you are in your plot and you do //sphere (or //hsphere) [the block you want the sphere to be] [radius]. The cmd should make the sphere around you, you being the center.
Example: //sphere glass 5
If that doesn't work, than can you please give us what the error message said when you tried it. Good luck :D