Why won't it save as .Mobi?

I have Calibre on my PC and I can save titles as .mobi with the option (right click on a title) Save To Disk -> Save only MOBI format to disk...

I wanted my wife to have the same program so I loaded Calibre onto her PC but the equivalent menu option says ePUB not Mobi ????? I have looked and looked and fiddled and played and cursed - but sll to no avail ...

HELP (Please).

Thanks.

Robert.