Unable to install theme

I downloaded the M Green Theme put the upgrade.lgu file in the root folder of the USB drive, connected the drive with the MediaNav evolution and set the ignition to position II. Unfortunately there is no option to update/ upgrade. Please let me know, how I can install the theme.
Thank you

if I add a detail to my first issue, which is a second one too: I bought and downloaded another theme and tried to install it on the MediaNav with the same result, it is not possible to install, it seems that upgrade.lgu is not recognized for installation. Can anybody help?