We are a couple in their 50's from USA, landing in Athens on May 9th. We need to be in Costa Navarino on May 13th. We have 4 nights to spend in Athens and to get there. After a week golfing Costa Navarino, we will return to Athens to go to the islands of Mykonos and Santorini.
I was thinking of spending 2 nights in athens May 9 - 11
What city should we go and spend another 2 nights on the way to Costa Navarino?

Nafplio is about half way in addition to being a highlight of the country. I am assuming you are driving. Could you put your time in Athens at the end of the trip, after the islands?
Remember you'll have to go all the way back to Athens for your flight--unless you can find flights out of Kalamata (I don't know).

I agree Nafplio would be a good choice for 2-3 nights, as it would give you a chance to see some of the UNESCO World Heritage sites in the area, and also be in a genuine Greek city before the modern Westin Resort at Costa Navarino.
http://www.visitnafplio.com/
Nafplio isn’t quite halfway, but there is a good divided highway most of the way from there to Kalamata. If you have the time, it would be worth seeing the archaeological site near Kalamata called Ancient Messene, and if you can break away from the golf course, near Costa Navarino are some beautiful seaside villages, Pilos, Methoni and Koroni.

Napflio sounds good and yes, we can stay in Athens at the end.

Sure, we can stay in Athens at the end. We can stay in Nafplio 2 nights, should we stay in Kalamata area too?