If you are running one of the earlier releases, please upgrade to this latest version of Unity of Command, by following this link and downloading the patch:
How do you know which version you're running: go to Main Menu, then Credits. The version number is noted in tiny letters at the bottom of credits (if there is no version number, it means you're running version 1.00).
NOTE to MATRIX GAMES and SLITHERINE customers: Matrix/Slitherine version has a slightly different installer and therefore a slightly different patch. Please consult Matrix and Slitherine support for this download. EDIT: since they are not making this available as of this writing, you can also grab it here for the moment.