Add this code:
.append("title")
.text(function(d) { return d.info; })
to the code where circles are created, as it is displayed here:
... and node "Heart -> ICD-10 -> I00 that contains "info" will get its tooltip!
Modified codepen is here.
NOTE: This will add a tooltip for circles. If you want tooltips for radially placed labels too, than you need analogous code for labels.