I can’t see this field in my post grid plug-in. Do you know what could be the reason? this is my website: http://www.2barch.it
Welcome to our forum.
If you are using free version then I am really sorry to say this feature is not available in free version.
If you using premium then first you need to select posttype and update settings you will see taxonomy and terms list under posttype.
You can select one taxonomy and unlimited terms to query post.