Restoring a SQL Server 2008 R2 backup on SQL Server 2005
You can't restore a 2008 backup on SQL Server 2005. Simply not possible.

You'll need to either:

(a) Restore it on 2008, then use Import/Export wizard, Generate Scripts Wizard, Copy Database Wizard, SSIS, 3rd party tools like SQL Compare, etc. to move the structure and data to the 2005 instance, or

(b) Upgrade. Developer Edition is $50 or so.