How to create custom action for list items using Client Object Model

Okay...You need to add custom action to list items using client object model.
This user custom action menu item will be added to drop-down menu which is displayed for list items.

          using Microsoft.SharePoint.Client;

            string urlWebsite = "http://xxxxxxxx:9999/";

            ClientContext clientContext = new ClientContext(urlWebsite);

            Web oWebsite = clientContext.Web;

            List oList = oWebsite.Lists.GetByTitle("TestList");

            UserCustomActionCollection collUserCustomAction = oList.UserCustomActions;
            UserCustomAction oUserCustomAction = collUserCustomAction.Add();
            oUserCustomAction.Location = "EditControlBlock";
            oUserCustomAction.Sequence = 100;
            oUserCustomAction.Title = "How to in SharePoint";
            oUserCustomAction.Url =  @"http://howtoinsharepoint.blogspot.in/";
            oUserCustomAction.Update();
            clientContext.Load(oList,
                list => list.UserCustomActions);
            clientContext.ExecuteQuery();

Location property specifies EditControlBlock, 
Sequence specifies an order of placement in relation to other user custom actions,
Url specifies an absolute path to a page that defines the action. 

Hope this helps...Happy Coding!!!!