I want to add 2 new fields to the master file description of a FOCUS database. How do I effect this new layout in the existing FOCUS database file?

you need to do a rebuild reorg

create your new master and run the following code.
oldfilename is the name of the old focus db, newfilename is the name of the new focus db
Once you have completed the rebuild rename the new master and database to the old name and your programs will continue to run with no changes.

USE
C:\TEMP\newfilename.FOC AS newfilename NEW
END
-RUN
REBUILD
REORG
DUMP
oldfilename
NO
-RUN
REBUILD
REORG
LOAD
newfilename
YES
-RUN