Download the release archive and extract it into your installation. The src/addons/MC/DropdownChooser/ directory should sit alongside the other add-ons.
Navigate to AdminCP > Add-ons and click Install next to [MC] Dropdown Chooser. The add-on stores no schema or per-user data; only its options and templates are added.
Visit AdminCP > Setup > Options > [MC] Dropdown Chooser. Decide whether Display thread count should be on, set the Search threshold to a number that fits your node count, and tune Search fuzziness if you have non-English forum titles or unusual punctuation.
Optionally pick forums to hide via Excluded forums. Their descendants are also hidden.