By default movies and TV Show added the last 7 days will show up in the New Category. If you want to extend this setting, modify the moviejukebox.properties file in the properties directory. And move moviejukebox.properties into the root yamj folder, that is one level back.
#Extend the New Category to 90 daysmjb.newdays.movie=90mjb.newdays.tv=90
More detailed info: http://code.google.com/p/moviejukebox/wiki/PropertiesConfiguration#mjb.newdays.*