If you are using the Analytics Runner to execute your tasks in lieu of the Task Service, logging is enabled by default and no action is required. The HRSTracelog.log is located in your Tasks folder.
To enable error logging, open the Telligent.Tasks.Service.exe.config file located where your task service is installed, and un-comment the system.diagnostics section:
<remove name="DefaultTraceListener" />
<add name="myListener" type="System.Diagnostics.TextWriterTraceListener" initializeData="HRSTraceLog.log" />
Then restart the task service. The next time any task runs, it will output a file in the same directory called HRSTraceLog.log.
This log file can grow very large, very quickly and so should only be enabled for brief periods of time as required for diagnosing problems with importing data.