日本語のドキュメントが意外となかったので
sudo sh -c "echo deb https://mega.io/linux/repo/xUbuntu_$((lsb_release -a 2> /dev/null)|grep Release | awk '{print $2}')/ ./ >> /etc/apt/sources.list"
curl -fsSL https://mega.io/linux/repo/xUbuntu_$((lsb_release -a 2> /dev/null)|grep Release | awk '{print $2}')/Release.key | sudo apt-key add -
sudo apt update
sudo apt install -y megacmd
参考
Installing megacmd in Ubuntu (console only) - ubuntuforums