CiviCRM 4.6.3 says no extensions available
You can control which extensions are displayed by overriding $civicrm_setting['Extension Preferences']['ext_repo_url']

try something like $civicrm_setting['Extension Preferences']['ext_repo_url'] = 'http://civicrm.org/extdir/ver={ver}|cms={uf}|status=';

obviously be careful that you install things that are compatible.