The /FLDQ/AD_MTCCODE is populated when you run the the Initial Match Code setup program which is basically the calculation of an index pool based on your match code (there are 5 types of match codes predefined in DQM. You could set it up in Substitution parameters into DS Designer).
(read before the 5.1.8 chapter in DQM user guide)
in the /FLDQ/RSADRINIBM program:
1) run first, Build the Index. it will create your index pool of BP (like your package of customers).
2) then run, Create Match Codes.
3) run Commit Dup Check Match Codes
and the duplicate check functionality will be running on your system (active it also in the customizing).
Optionally, you could run the Perform Batch Match option, to perform a massive searching of duplicates on your system (is like the quarterly adjustment for normalization all your addresses, but in this case, for searching duplicates on your system).
Regards.