AppleTV sees my Mac Mini but It can't access its information
From the Apple Discussion forum:

In iTunes, Edit&gt;Preferences&gt;Sharing&gt;Share my library on my local
network.
I saw this was unchecked, and checked it.

And maybe more meaningful:

&quot;Once Home Sharing has been turned on, Apple TV will automatically
find iTunes libraries on your local network that have Home Sharing
turned on using the account name you entered. The shared libraries
will appear in Computers on the main menu of Apple TV.&quot;
The implication is that a &quot;shared library&quot; must exist on the computer
that has Home Sharing turned on in order to be found.

And to conclude:

First things first, unplug the ethernet cable from your apple TV and
check your settings under Setting &gt; General &gt; Network. Note the IP
address.
Secondly, plug the ethernet cable back in and check your
settings. Setting &gt; General &gt; Network - are they the same or totally
different? If they are different you most likely have an issue where
your iTunes Homeshared Library is running off of a different IP
Address than your Apple TV.

Also, make sure you have your ports right!