If you have a .ENG file all ready, put it in your "DATA" directory under the RockSim folder, and run the "Compile Engines" program. Tell it which .ENG files you want, click the "Compile" button, and that's it.
If you're just dealing with a text file, and it's in the standard "RASP" format, you can open or create a .ENG file in notepad, then cut and paste the required text into the file then save it with a .ENG extension. Then, run the "Compile Engines" program as described above.