Do you want to popup the infoWindow for one feature when the map is loaded?
If so, as to the cluster sample, you can invoke a function, like queryMyFeature, near the end of function addClusters. queryMyFeature will query a feature or features that you like to display the infoWindow. Then inside the callback function of the query, you can use
var popLocation = response.features[0].geometry;
map.infoWindow.setFeatures(response.features);
map.infoWindow.show(popLocation);