Using the Data Migrator DMC, I followed the tutorials in the users guide for the DB_LOOKUP function, and it resulted in the following error:

Issuing PREPARE
Request will process data via NON-Pass Through (NON-APT)
HOLD file will be created for output file named: SQLIN.
(FOC1400) SQLCODE IS -1 (HEX: FFFFFFFF)
(FOC1407) SQL FETCH CURSOR ERROR. : COMPINFO
(FOC1400) SQLCODE IS -1 (HEX: FFFFFFFF)
(FOC1407) SQL FETCH CURSOR ERROR. : INVINFO
...RETRIEVAL KILLED
(FOC026) THE REPORT IS NO LONGER AVAILABLE
Return Code = 18708

... the problem is reproducable only when trying to include and insert the column looked- up into the target table. DB_LOOKUP would be very useful to my development, but I'm about to abandon it as a broken feature.

A 18708 message isn't really an error. It's a warning saying there weren't any records returned. Which I'm going to guess is a result of the SQL FETCH CURSOR ERROR messages you received. Make sure you have access to COMPINFO and INVINFO.

If you want to stop the 18708 message, open your data flow, right click anywhere on the canvas, click in PROPERTIES, click on EXECUTION, and uncheck "STOP PROCESSING IF 0 ROWS SELECTED". This won't stop the other error messages in your case. What this is good for is in situations where getting zero records to load is a valid scenario and you don't want the 18708 to trigger some sort of error processing.

For your particular problem, if you confirm that you have access, I recommend opening a case with IBI.

Mostly ignorant of Data Migrator, but DB savvy. What made my pause about the problem description was the fact that a DB_Lookup (select in my mind) function was being used to insert data. Is there a separate function that should be used to insert?

I guess I am asking mostly for my education, but maybe it will help others.

DB_LOOKUP itself doesn't do an insert. In the first tutorial example it would do something equvialent to SELECT STORENAME FROM DMCOMP WHERE STORE_CODE = ? where the input values are substituted for each row and the result is assigned to the specified name.

That implies you need to be able to read from DMCOMP; can you do a Sample Data on that table? If not you may need to recreate the sample procedures and data for DataMigrator.