Ok, had the same problem. This fix worked for me.
Edit your ngg_dkret3.css file (in the css folder of the plugin).
Change this:
.ngg-gallery-thumbnail span {
display:none;
}
To this:
.ngg-gallery-thumbnail span {
font-size:9px;
padding-left:5px;
display:block;
}
Then it will work, but the styling may be a little out of whack so you may need to fiddle with that a bit.
If that doesn’t work for you you may want to check the other .css files in that folder to see if any others have display: none; and change them accordingly.