If SQLExpress LocalDB is already installed (and it sounds like it is) all you need to do is supply an appropriate connection string:
Fx: "Server=(LocalDB)\v11.0; Integrated Security=True;"
For other variations and a discussion of the subject check What is the connection string for localdb for version 11
Personally I would uninstall any old SQL Server product editions, then any new ones, and ultimately re-install SQL Server 2012 from this URL: http://www.microsoft.com/en-us/download/details.aspx?id=29062
This will ensure you have an up to date development environment, 2008 R2 is pretty dated and it sounds like you have some side-by-side installation of SQL2k12 and SQL2k8 components, seems a little dirty. The SQL Server 2012 Express download page will offer you access to the latest versions of SSMS, x64 editions as well as "full" editions (e.g. full-text search, SSIS, etc.)
Most people install 2008 editions because they don't know how to locate SQL Server 2012 Express, which is a little strange. Probably due to Web PI not being updated for so long.