On a Rasberry Pi 2, if you need packages which is not includes inside Raspbian repository, you can use debian repository (Wheezy for now).

Use these command lines to add Debian Wheezy repository

echo "deb http://ftp.debian.org/debian/ wheezy main" > backports.list
echo "deb http://ftp.debian.org/debian/ wheezy-backports main" >> backports.list
sudo mv backports.list /etc/apt/sources.list.d

When it’s done, try to make a sudo apt-get update. Usually, you can see that APT need to register the new repositories and request to add public keys. First install a list of key server,

sudo apt-get install debian-keyring

Next register the keys provide by APT (replace the key 8B48AD6246925553 by yours),

gpg --recv-key 8B48AD6246925553
gpg -a --export 8B48AD6246925553 | sudo apt-key add -

and then update your packages list,

sudo apt-get update

Note: If you need a package in a specific repository used this kind of command

sudo apt-get install -t wheezy-backports gstreamer1.0*