Syntax problem

In GAUSS every statement needs to end with a semi-colon. You are missing the semi-colon after print "Unusually low". Since there is no semi-colon to end this statement, the statement continues with the next line and you end up with one statement like this:

print "Unusually low" elseif 0 < meanx and meanx < 0.66;

That is not a legal statement in GAUSS. Here is the corrected version with 3 missing semi-colons added:

x=rndu(100,1);
meanx=meanc(x);
if meanx<0.33;
   print "Unusually low";
elseif 0 < meanx and meanx < 0.66;
   print "Just right";
else;
   print "Unusually high";
endif;

thanks a lot