My Mac mini's iTunes library does not appear in Home Sharing on my Macbook Air
Go into Preferences and make sure you have Shared Libraries ticked in the Show section of the General Preferences pane.

You might also have the details hidden in the sidebar. If the sidebar has SHARED in grey writing with nothing under it then try placing the cursor to the right of the word (near the right edge of the pane) and either Hide or Show will appear, allowing you to toggle the list.