You are right there is no Home Sharing icon in the default view for iTunes 11
View > Show Sidebar on a Mac
Ctrl S on windows
If the homeshare is turned on you should see it in the lefthand sidebar if you are sharing between computers. If you have one computer and multiple devices you will not see it because they just stream your content.
I don't know if it is your Airport that is the issue as I do not currently have one just a BT Hub 3 wireless router
If you have not already tried it try logging out of home sharing on all devices
on the computer File > Home Sharing > Turn Off Home Sharing
On the devices, Settings, Music scroll down and tap on the home sharing to sign out.
Re activate Home Sharing by typing in the same Apple id
On the computer File > Home Sharing > Create Home Share
This may just reset everything