How I label scatter plot points in Numbers?
As of Numbers 3.6 on OS X 10.11, this is not currently possible. 

A few alternatives:


Manually add text above each individual point, though obviously a huge hassle.
Use Excel (if you have access), as this functionality appears to be supported.
Use Google Sheets (free), which supports this feature.


Using a Google Sheet is definitely the most cost effective method of the three listed above, and you should be able to easily migrate your Numbers sheet by copying and pasting cells, or by exporting as an .xls file, and then uploading to Google.