Gah! I thought I had tried everything (single/double clicking on a workout, selecting ‘show details’, changing to week/month view, etc.), and never saw a window like you show there. I spent another few minutes at it just now and finally realised that clicking on the name of the workout brings up that window. (And I only discovered that accidentally, after trying to click every available are within the box showing the workout).
Thank you for sharing that screenshot, I needed something concrete like that to convince me that the “Add Data” option still existed.
And a suggestion for Athletica: for me this change to the UI was entirely non-intuitive, so please make this information available somewhere. I had checked your support docs previously but found nothing to tell me that the actual name of a workout is now clickable text. Previously I always clicked on “…” in the top corner of the workout box and “Add Data” was one of the options presented directly there, so this new procedure is a significant change from before.