It seems your database has been created with SQL Server 2008, but the actual version of SQL Server you are trying to attach it to is SQL Server 2005 (see
Internal SQL Server Database Version Numbers[
^]).
The help link shows to you may use SQL Server Management Studio 2008 R2 (10.50); are you sure that the
instance you are trying to attach your database to is also a SQL Server 2008 R2 one? Because the error message states this instance is a SQL Server 2005 one.