I am not familiar with the Flickr WordPress Plugin but in order for a Juicebox (or WP-Juicebox) gallery to find and display Flickr images, you'll need to group them together somehow, using one of the suggestions below with the Juicebox configuration options in bold.
(1) A Flickr account (using flickrUserName or flickrUserId).
(2) Tagged images within a Flickr Account (using flickrTags and flickrTagMode).
(3) A Flickr Set (using flickrSetId).
(4) A Flickr Group (using flickrGroupId).
Short descriptions of all these Flickr configuration options can be found in the Lite Options and Flickr Pro Options sections of the Config Options page.
If using WP-Juicebox (the dedicated Juicebox Plugin for WordPress), then Lite Options have their own individual interface controls but Pro Options must be entered as text in the Pro Options text area of the gallery settings window, one per line, in the format:
Also, if using WP-Juicebox, be sure to set "Flickr Account" as the "Image Source", otherwise, set useFlickr="TRUE" in your gallery's settings.
I hope this helps.