You can get the latest release from https://github.com/chevdor/nrs-dividends_scanner/releases
Unzip the file, rename the folder to dividends_scanner and copy it to:
C:\Users\your.name\AppData\Local\Nxt Wallet\nxt\html\ui\plugins
or
/Applications/Nxt Wallet.app/Contents/Resources/nxt/html/ui/plugins
Then start your client and ensure to enable plugins.