Thanks. I fixed the license in accordance with https://github.com/Ubuntu/gnome-shell-extension-appindicator/blob/7863517511427747175bb2a28a7d4aa5320a22f4/appIndicator.js#L3-L6.

@jonafato ... hi is there some specific way to setup this extensions. Have it installed on the latest NixOS 18.09 and no icons ever show

@goodwillcoding Did you enable it in GNOME Tweaks?

Alternately, you can add something like the following to your configuration.nix but then you need to make sure the dconf database does not set the key (dconf reset /org/gnome/shell/enabled-extensions) since this just overrides the default value.

services.xserver.desktopManager.gnome3.extraGSettingsOverrides = ''
  [org.gnome.shell]
  enabled-extensions=['dash-to-dock@micxgx.gmail.com', 'nohotcorner@azuri.free.fr', 'appindicatorsupport@rgcjonas.gmail.com', 'GPaste@gnome-shell-extensions.gnome.org', 'mediaplayer@patapon.info', 'ubuntu-appindicators@ubuntu.com']
'';

Last, you could store the value in a dconf db declaratively, but that is more involved at the moment: #42053 (comment)

@jtojnar yeah, turned out it worked just fine, but the 4 programs I tested it with tried can only be show by Top Icon (my guess they were either compiled with appindicator support or have some weird one or the other logic [blah, blah Unity style]) ... my apologies for bugging ya