When you open the options for SLIDES, you can specify a title for your slide in the Text field and you can place your description in the field below it. You can easily just copy and paste your description into the field.
If your description has multiple paragraphs or if you want line breaks, you need to make sure that when you place your paragraphs into the description box, get rid of any hard returns (or spaces) between your paragraphs.
Here is an example of paragraphs with hard returns:
Example (Showing inside the widget's options panel):
A "hard return" like this will cause the gallery not to load properly, or not load at all.
Therefore, to create line breaks between your paragraphs, type <br><br> between your paragraphs like this:
If you don't get rid of the hard returns (spaces between paragraphs), then the widget will break. It's easiest to edit your text in a text editor and then copy and paste it into the widget.