For oh-my-zsh, clone it into your custom/plugins directory like so:
git clone https://github.com/fuzzylogiq/autopkg-zsh-completion $ZSH_CUSTOM/plugins/autopkg
Let oh-my-zsh know this is a plugin:
touch $ZSH_CUSTOM/plugins/autopkg/autopkg.plugin.zsh
And then add autopkg to your plugins in .zshrc
plugins=(autopkg ...)
If you use zgen, add the following line in your .zshrc with your other zgen load commands.
zgen load unixorn/autoupdate-zgen
You'll probably want to reload your shell:
exec $SHELL -l