Ubuntu Tweak does not launch

Distribution: Ubuntu 10.04.1 LTS
Application: Ubuntu Tweak 0.5.6
Desktop: gnome

Traceback (most recent call last):
  File "/usr/bin/ubuntu-tweak", line 109, in <module>
    from ubuntutweak.mainwindow import MainWindow
  File "/usr/lib/python2.6/dist-packages/ubuntutweak/mainwindow.py", line 148, in <module>
    MLOADER = ModuleLoader(modules.__path__[0])
  File "/usr/lib/python2.6/dist-packages/ubuntutweak/modules/__init__.py", line 28, in __init__
    package = __import__('.'.join([__name__, module]), fromlist=['modules'])
  File "/usr/lib/python2.6/dist-packages/ubuntutweak/modules/updatemanager.py", line 22, in <module>
    from aptsources.sourceslist import SourcesList
  File "/usr/lib/python2.6/dist-packages/aptsources/__init__.py", line 4, in <module>
    apt_pkg.init()
SystemError: E:Abriendo fichero de configuración /etc/apt/apt.conf.d/99synaptic - ifstream::ifstream (13: Permiso denegado)