tt-rss has been installed into ${INSTDIR}

You should point this to the DocumentRoot of your web-server:
   # ln -s ../piwik /var/www/htdocs/piwik
(make sure you use a relative symlink since Apache is chrooted)

and proceed to complete the installation by reading the online
documentation. Which can be found at:
http://tt-rss.org/trac/wiki/InstallationNotes

You can ensure you have a working install by accessing:
http://<localhost>/tt-rss/