See: How to: Attach a Database File to SQL Server Express
Login to the database via sqlcmd:
sqlcmd -S Server\Instance
And then issue the commands:
USE [master]
GO
CREATE DATABASE [database_name] ON
( FILENAME = N'C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Data\<database name>.mdf' ),
( FILENAME = N'C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Data\<database name>.ldf' )
FOR ATTACH ;
GO