How to add a surge protector on 240 circuit at pool sub panel - neutral or ground connection?
Good question! DOC - to answer your question, (for 240 volt application) the two BLACK wires go to a 2-Pole 20A breaker and the white goes to the ground bus or a bonded neutral bar.  Here is a cut sheet for a unit similar to yours (and similar to those that we install on a daily basis:

http://www.five-two-one.com/pdf/LT521SPD_Web.pdf

Hope this helps you!