Are you using UberMenu 3? Please see the UberMenu 3 Avada Integration Instructions
Avada 3.x
In Avada 3, the UberMenu option still appears in Appearance > Theme Options > Extra Options > Enable or Disable Support for Plugins Installed Manually > UberMenu Plugin Support, it is just located slightly further down in the settings
Avada 2.1.1
ThemeFusion added UberMenu support into Avada in version 2.1.1 of the theme. It can be activated in the Avada Theme Options Panel.
