NA-iTTL (NIKON.... according to their web site... the answer is Yes, it's compatible with Nikon's iTTL mode...
Their vendors B&H Photo, and Amazon explicitly state it's iTTL compatible on their web sites...
Having said the above... and the fact that you've owned the device for quite some time, it wouldn't hurt to send an email to their technical support folks with your exact model/revision numbers and ask them for clarification...
Of course, the simplest and quickest thing you can do is simply attach it to the D5100 and see if it works...