get the signing key:
curl http://apt.basho.com/gpg/basho.apt.key | sudo apt-key add -
add the repository to your system:
sudo bash -c "echo deb http://apt.basho.com `lsb_release -sc` main > /etc/apt/sources.list.d/basho.list"
then install Riak
sudo apt-get update
sudo apt-get install riak