Hello,
the Database Documentation page describes how to connect to a database in KNIME. The driver must support at least JDBC 4.0. If the JDBC driver requires additional files such as dlls you need to make sure that they use the same architecture as your KNIME installation e.g. 64/32 bit.
For MS SQL you can use the Microsoft JDBC Driver 4.0 for SQL Server which should also work for MS SQL Server 2012.
Bye,
Tobias