The style is already set to include the DOI. You just don’t have them in your Zotero library. Google Scholar isn’t a good source for article metadata—among other things it doesn’t provide the DOI. You should always click through to the publisher webpage and import from there.
For your existing items, install the DOI Manager plugin and use it to retrieve the DOIs.
For your existing items, install the DOI Manager plugin and use it to retrieve the DOIs.