It's best to follow the instructions on this HOWTO on the Gentoo Wiki.
When installing mt-daapt it is not necessary to use the howl USE flag, simply execute:
sudo emerge mt-daapd
Now copy the example file (/etc/mt-daapd.conf.example) to (/etc/mt-daapd.conf), and edit the following values to read (feel free to change these values, especially passwords, etc):
web_root /usr/share/mt-daapd/admin-root port 3689 admin_pw A_PASSWORD # the password used for admin pages db_dir /var/cache/mt-daapd mp3_dir /music # the directory where the music is stored servername gentoomusic # the server name the itunes client sees runas nobody playlist /etc/mt-daapd.playlist password password # This is the password itunes will prompt for when a client attempts to connect extensions .mp3,.m4a,.m4p,.ogg rescan_interval 300 logfile /var/log/mt-daapd.log
You can run the server using:
sudo ./mt-daapd start
To add mt-daapd to the startup scripts execute the following:
sudo rc-update add mt-daapd default
On a different machine (within the same subnet) you can open iTunes, search for music, and you should see gentoomusic. If you enter the password (password in this example) you should now see all of your music :)