The configuration of the external IDM authentication/synchronization is done in the same way as the regular properties. There is a properties file named activiti-ldap.properties in the WEB-INF/classes/META-INF/ folder in the WAR file. The values in a file with the same name on the classpath have precedence over the default values in the former file.
In addition, in the same folder, the example-activiti-ldap-for-ad.properties file contains an example configuration for an Active Directory system.