Maybe I'm on thin ice here, since I haven't done any coding (yet) with FCMP.
But looking into some examples in the online doc, maybe I could suggest something anyway...
It seems that if you are going to use the return() function, I think you are supposed to use the function name as argument, and also using the function name in the assignment statement as well. see:
http://support.sas.com/documentation/cdl/en/proc/59565/HTML/default/a002890491.htm
In your case, this would be:
proc fcmp outlib = sasuser.FCMP_Cat.fcns;
function SUMM(x,y);
summ = x + y ;
return(summ);
endsub;
quit;
Hope this helps...
Linus
Data never sleeps