At this time, Grammarly does not offer an add-in for MS Word or Pages for macOS users.
However, you can go to the Apps section in your Grammarly editor and download a native desktop app for Mac. Also, you can use Grammarly’s browser extension for Safari, Chrome, or Firefox on a Mac.
To preserve the formatting of your text while using Grammarly's online editor, please use the Upload and Download buttons to check documents written in other programs. Once you're finished editing your text in the editor, clicking the Download/Export button will send the edited text back to its source document, preserving the original text formatting (font, highlights, spacing).
Supported file types are MS Word (.doc, .docx), OpenOffice (.odt), .txt and .rtf