@DaisukeJigen,
bonkaholic’s issue turned out to be this:
if(window.location.href == "https://www.wanikani.com" || window.location.href == "https://www.wanikani.com/dashboard"){
The location was actually https://www.wanikani.com/ (i.e. with a slash on the end), so it wasn’t installing the menu link.
You can use pathname instead of href, since it automatically adds a "/" if one isn’t present:
if(window.location.pathname == "/" || window.location.pathname == "/dashboard"){