terajni.dll was a component of the Type 2 Teradata JDBC Driver, which hasn't been supported for a very long time. That file is not used by, or shipped with, the Type 4 Teradata JDBC Driver.
Instructions for how to use the Teradata JDBC Driver with WebSphere are available in the Teradata JDBC Driver Reference.
http://developer.teradata.com/doc/connectivity/jdbc/reference/current/WebSphere.html