Seems like this must be so simple no one wants to answer it. I realise technical questions are more compelling but I’ve spent hours already on this, searching forums, watching youtube videos and the answer is not that apparent.
@hinemoa this plugin has a lot of requests so you’d be more patient, only 6 hours had passed from your first request and in the meantime you had better look at the plugin options more than watching videos or something else… maybe you did but you don’t say.
So now, after setted up the gallery, have you in Manage galleries > Gallery written your description in the corresponding field?
The first is for the alt & title attribute, the second is for a description/title/name under the image.
The latter will be displayed in gallery thumbs if you do one of these things:
For the single gallery, post/page visual editor > Nextgen icon > customize the display settings > template > gallery-caption.php
For all of the galleries, Gallery settings > basic thumbnails > template > gallery-caption.php
For the images opened with the effect, all of them but highslide display that description, so check if you have choosen one of the five other effects.
