I have installed skype on ubuntu 15.04 and the skype icon doesn't appear under applications so to fix the issue install the sni-qt package.Sni-qt is A Qt plugin which turns all QSystemTrayIcon into StatusNotifierItems (appindicators).The reason for this is skype/pidgin installation relies on the 32 bit version of sni-qt.
Open the terminal and run the following command
sudo apt-get install sni-qt sni-qt:i386
After installing above package you need to restart skype/pidgin application to appear icon.
Thanks. Worked for me.