[Plugin: NextGEN Gallery] How to center the slideshow

I had the same problem and solved it by adding code. I found this somewhere and pasted it into wordpress > APPEARANCE > EDITOR.
Worked for me

.nleft { float: left; font-size: 1%; }
.nright { float: right; font-size: 1%; }
.fancy-letter { font-size: 350%; float: left; padding: 0.2em 0.2em 0.2em 0; }
.gallery-leading { width: 100%; position: relative; float: left; text-align: center;}
.gallery-contents { position: relative; float: left; padding: 0 8px;}
.gallery-contents.left { margin-left: -100%; }
.gallery-contents.below { width: 100%; display: inline-block; }
.gallery-container { position: relative; }
.gallery-photo-counter { float: none; font-style: italic; width: 100%; text-align: center; display: inline-block;}
.gallery-thumbs { text-align: center; }
.gallery-thumbs img { float: none;}
.image-container { text-align: center; margin-bottom: 1em;}
.exif-button { float: right; border: 1px solid; padding: 0 1em 0 0.8em; }
.exif-button a { text-decoration: none; line-height: 1.2em; padding: 0.3em 0; display: inline-block;}
.exif-button .icon { width: 16px; height: 16px; background-repeat: no-repeat; display: inline-block;  margin-right: 0.5em; }
.exif-panel { display: none; }
.navigation-attachment { width: 100%; display: inline-block; border-top: 1px dotted #c0c0c0; margin-top: 1em; }

/* Gallery Fixes */
.ngg-slideshow {
   width: 100% !important;
}
.ngg-slideshow * {
    margin: 0 auto !important;
}
.ngg-slideshow img {
    position: relative !important;
    display: block;
}

Great, I added it to the nggallery.css file and it is working fine.

Thx!!