Hi Olfa,
welcome to the forum!
You simply cannot get a connection to the MiKTeX repository. It could be an Internet setting, a proxy setting on your computer, or the default or chosen repository is not available. From jon
here: Open MikTeX 2.9 >> Maintainance >> Settings. Click on the Packages tab and change the package repository to a nearby host. Check
http://miktex.org for available repositories.
Stefan