Keep in mind you won't be able to use this functionality until the connectivity test result returns as a success, with this green connected sign being displayed.
You can then select a group of files from which you want to download missing tag data and press the download button. The download progress will be shown over the Tag data, as seen below. This button is available both on top of the Tag Data View, and in the context menu on the list, by right-clicking, and selecting the "Download Missing Tags" Option.
Often, the data returned will include multiple recordings, and the first option, though sorted, might not be your preferred option. If this is the case, use the navigation arrows to cycle between recordings.
Don't forget to save once you have found the right data for your file!