Just to give you a shortcut on that, there is another problem that causes problems when installed to /usr/local.
The sample config file has everything pointing to /usr (/usr/share/mt-daapd/admin_root, for example), but if you didn’t configure with –prefix=/usr, then they are in /usr/local/share/mt-daapd/admin_root.
So double check that. Also, make the db_dir (/var/cache/mt-daapd) and make sure it has read/write permissions for the runas users. Lastly, verify that the mp3_dir has at least read permissions for the runas user.
That should do it.