Comment 4 for bug 1347128

The problem here is that owncloud-client doesn't actually produce an indicator, but rather an old-style systray icon. Until now this wasn't a problem because a Qt plugin called sni-qt[1] would automatically convert systray icons to indicators. Unfortunately, sni-qt doesn't support Qt5[2], and between the trusty and utopic versions owncloud-client switched from building against Qt4 to Qt5.

There are a couple of possible solutions:
1. Try to build the current version of the client against Qt4
2. Update sni-qt to support Qt5
3. Patch in support for AppIndicator[3] or StatusNotifierItem[4][5] directly

The sni-qt project seems to be dead, with no updates since Quantal (12.10) so 2. seems unlikely without some community help.

[1] https://launchpad.net/sni-qt
[2] http://askubuntu.com/a/159232
[3] https://unity.ubuntu.com/projects/appindicators/
[4] http://www.notmart.org/misc/statusnotifieritem/index.html
[5] http://api.kde.org/frameworks-api/frameworks5-apidocs/knotifications/html/classKStatusNotifierItem.html