Installing CiviCRM 4.6.11 on a Joomla 3.4.8
CiviCRM is likely much heavier then the average Joomla Extension. And because it runs behind more CMS-systems the preferred installation could be different then the usual Joomla Extension.

Please have a look at the "Installing CiviCRM for Joomla" page on the CiviCRM site and follow the instructions there for the best way to install CiviCRM for Joomla:

http://wiki.civicrm.org/confluence/display/CRMDOC/Installing+CiviCRM+for+Joomla