There are some issues if you connect to SS2012 (Subscriber) with SQL Management Studio 2008 or 2008 R2 (Publisher/Distributor).

Try with stored procedure or with SQLCMD

DECLARE @publication AS sysname;

DECLARE @subscriber AS sysname;

DECLARE @subscriptionDB AS sysname;

DECLARE @frequency_type as int;

DECLARE @subscriber_security_mode as int;

DECLARE @subscriber_login AS sysname;

DECLARE @subscriber_password AS sysname;

SET @publication = ______________ ;

SET @subscriber = ________________;

SET @subscriptionDB = _______________;

SET @frequency_type= ___ ; --(you must check frequency type)

SET @subscriber_security_mode= ___ ; --(you must check security mode)

SET @subscriber_login=______________;

SET @subscriber_password=__________; --(if security mode is 0, SQL Server authentication)

--Add a push subscription to a transactional publication.

USE [DBName]

EXEC sp_addsubscription

@publication = @publication,

@subscriber = @subscriber,

@destination_db = @subscriptionDB,

@subscription_type = N'push';

EXEC sp_addpushsubscription_agent

@publication = @publication,

@subscriber = @subscriber,

@subscriber_db = @subscriptionDB,

@subscriber_security_mode=@subscriber_security_mode,

@subscriber_login=@subscriber_login,

@subscriber_password=@subscriber_password,

@frequency_type = @frequency_type,

@job_login = ___________,

@job_password = __________;

GO