That's ArcScene, not ArcMap, and as such has no real labelling abilities. However there are plugins.
See this plugin for one potential solution: http://edndoc.esri.com/arcobjects/9.0/Samples/3D_Analyst/Visualization/ArcScene/Text_Toolbar/LABEL_3D_TOOLBAR.htm
and https://geonet.esri.com/thread/96511
And also this answer: How to easily label point values in ArcScene?How to easily label point values in ArcScene?