You should be able to just double click the Assist icon and it should bring up the feedback. If it doesn't then somehow things didn't get installed correctly, you may have to do a reinstall of Yosemite. Lots of luck getting any kind of help from Apple, they are hard to get in contact with. Check your applications folder to see if the feedback app is there, possible the link from the desktop icon to the app is broken and you may be able to start it from the app itself. If so you can always delete the desktop icon and make a new one.