You can set the ClientSettings.Selecting.AllowRowSelection property
in order to enable client-side selecting of RadGrid items with a mouse click. If you want to handle the client-side select event, set your RadGrid's ClientSettings.Events.OnRowSelecting property to the name of a method that will handle the event.
Take a look at this help topic
which discusses how to enable client-side selecting with the click of a mouse, and this help topic
which discusses how to handle the client-side OnRowSelecting event.
I hope that helps. Let me know if you have any further questions.