The Riak KV installation using apt on Ubuntu documents links are all broken on the documentation page.
Any ideas on how to do an install?
Download the package for your favourite version of Ubuntu from https://files.tiot.jp/riak/kv/3.0/3.0.9/ubuntu/ for the latest release at time of writing of KV 3.0.x or from https://files.tiot.jp/riak/kv/2.9/2.9.10/ubuntu/ for the latest release at time of writing of the KV 2.9.x series. If your heart is set on a 2.2.x version (based on the link from the question) then they can be found at https://files.tiot.jp/riak/kv/2.2/
Follow the instructions on the latest version of the docs which you can find at https://www.tiot.jp/riak-docs/riak/kv/2.9.10/setup/installing/debian-ubuntu/ (basically sudo dpkg -i /path/to/riak/debfile but you can remove it later with sudo apt remove riak). The version 3.0.x docs will be released soon but the installation process is the same. Just be aware that for 3.0.x all riak-admin and riak-repl commands should have the hyphen replaced with a space i.e. riak-admin -> riak admin.
Although files.tiot.jp does support being used as a rhel compliant repo i.e. yum install riak-3.0.9 is possible after configuring it as a repo, we have not added repo based apt support yet as that is still WIP.