All static calibration files necessary to reduced FORS2 data are included in the calibration directory. In Reflex, these are located in install/calib/fors-versionnumber/cal folder. The directory is also included in the FORS2 pipeline release file (available for download from http://www.eso.org/pipelines). The *ACAT* files are fits tables which contain the master line catalogs. Which of these files to use depends on the grism and order blocking filter used in your observation. This information can be extracted from the header of the fits file of the science data.