C# code to execute custom workflow in MSCRM 2011:

ExecuteWorkflowRequest request = new ExecuteWorkflowRequest()

{

    WorkflowId = _workflowId,

    EntityId = _leadId

};

Console.Write("Created ExecuteWorkflow request, ");

// Execute the workflow.

ExecuteWorkflowResponse response =

    (ExecuteWorkflowResponse)_serviceProxy.Execute(request);