I'm having the same issue, when I just went in to try to set up the plugin after installing it, I put everything the way I wanted it in the settings menu and got the message:
"Invalid values, the settings have not been updated
API Key is not valid"
I also tried to just put the code: [flickr_photostream user_id="xxxxxxxx"] directly into the text tab of the new wordpress page that I created, and got the message : "FlickrPhotostream error: The lightbox attribute can be only "none", "colorbox" or "swipebox"."
I'm trying to display my flickr images from this flickr account: http://www.flickr.com/photos/114623407@N06/
to this web page:http://www.faithandaction.org/web/photo-stream/
that last error could just be because I can't save the settings because my api key won't work, but I"m not sure. I'm not much of a coder, so I'm not sure I would know how to activate cURL, or what that is. Thanks for any help that you can give.