Attach SQL Server 2008 R2 on SQL Server Express 2008
You can't attach a SQL Server 2008 R2 database to a SQL Server 2008 or 2005 instance. You can only go up, not down; you can't make your 2008 R2 database compatible with previous versions. The solution is to remove your Express instance and replace it with a 2008 R2 instance, then you will be able to attach your database...