Another possible solution:
Go into the application folder (right click in Impulse, select 'Open application folder'), and there are three different 'Mesh' folders. The first is in the root folder, and there may be two more, depending on which version of the game you have, in the \Entrenchment and \Diplomacy folders. Delete everything in those folders, then right click on the game(s) in Impulse, and select 'Verify Installation'. Impulse will check the integrity of the installed application, and an update should be made available in Impulse. This is not an update to a newer version, but simply Impulse using the update mechanism to replace the missing files. Once you complete the update, reboot your computer.