On your Mac it is in the iTunes File Menu
File > HomeSharing > Turn On home sharing
Unless you activate it on another computer you will not see an icon for HomeSharing. Even if devices are linked to it they do not visibly show on your Mac. It should appear in the Left Hand Sidebar. View > Show Sidebar if you do not already have it switched on.
On your iDevices you can access the Home Sharing by going into Music and selecting the more option. Then Tap on Shared and select the library you want to share from