how to display value 123456 value in the 1,23,456 fromat in Crystal report

One option is to use formula fields as given here
http://khanrahim.wordpress.com/2011/04/25/customized-thousand-separator-in-crystal-report/[^]

Second option is set the current culture to a culture with above system of comma separation say hi-IN (please check the format in Crystal Reports) to the Crystal Report

CrystalDecisions.Shared.SharedUtils.RequestLcid = "Country Code";

please check the above from your Crystal Reports documentation to set the current culture.
I do not have Crystal Reports to test.
In the crystal report set the thousand separator as shown here
http://support.insightdirect.com/help/cr/frmtnumcr.htm[^]
I think this may work. But please try.