At first I was puzzled about where/how to install but what worked for me here was to simply double-click the download (after double-clicking the zip). This installed
Apply Keyword Styles.omnioutlinerjs in the the bowels of the Library at: Library > Containers > com.omnigroup.OmniOutliner5 > Data > Library > Application Support > PlugIns.
Apply Keyword Styles then showed up in my Scripts menu (per the screenshot above).
I understand we need to wait until 5.0.5 build 288342 (or higher) for this to work.