Ok, from the top.
1) make a new folder, say c:\lisp-dan
2) add that folder to the ACAD "Support file search path"
3) unzip file from lilly pond in new folder (c:\lisp-dan)
4) load lisp file into ACAD (load "dimsuffix")
5) in ACAD type dimsuffix and follow prompts
the dimsuffix program will load the .dcl file so long as it's in the ACAD search path.
You can add your code to the lisp but, it has to be in the right place. I tried to give you something to work with and learn from, if it gets the best of ya just yell.