The UCAC number is the zone number (South Polar Distance in degrees by 1/2's)*2 followed by the six-digit sequence number of the star within the RA-sorted zone. This number will change with each release of the UCAC catalog.
Before you can use this program, the path to the directory containing the region files must be set. The default is to use the value of the variable ucac2path in the source code file libwcs/ucacread.c, which is initially set to /data/astrocat/ucac2, which is where the catalog resides at the Center for Astrophysics. If you do not have access to the source code and need to change the path, you can set the environment variable UCAC2_PATH to the directory containing the region files, and that will override the file setting.
The minimum command line is imucac2 FITS or IRAF filename
The same effect can be obtained by running imcat -c ucac2 FITS or IRAF filename