Attach the following code as a .css file in the Resources section of the in5 dialog (how to attach resources):
.anythingSlider-in5 { position:absolute; top:0;bottom:0;left:0;right:0; margin:auto !important; }
To vertically center a Flipbook, see:
https://ajar.freshdesk.com/en/support/solutions/articles/26000035585