A couple of suggestions: functions like rem look better infix:

     x `rem` 3

this means the same thing but is more readable (I think).

Instead of folding, just map putStrLn over the list:

    mapM_ (putStrLn . show') [1..100]

mapM_ is just a version of map for monads that ignores its result (it returns m () instead of m a). You can also use forM_ which is mapM_ with its arguments reversed.

Finally, I think this site lets you format code by putting four spaces in front of each line. I hope :)

Edit: One other thing. You don't need all those parentheses in the first line.

    rem x 3 == 0 && rem x 5 == 0 = "FizzBuzz"
    x `rem` 3 == 0 && x `rem` 5 == 0 = "FizzBuzz"

Both versions should work and I think they are easier to read. I could see (rem x 3 == 0) also being clear, but the outermost parentheses do not help at all.

Here's a shorter version:

  fizzbuzz x | x `mod` 3 == 0 && x `mod` 5 == 0 = "FizzBuzz"
             | x `mod` 3 == 0 = "Fizz"
             | x `mod` 5 == 0 = "Buzz"
             | otherwise = show x

  main = putStr $ unlines $ map fizzbuzz [1..100]

For us old-school types, here's the 60-second version in C:

    int main(int argc, char ** argv) 
    { 
    	int i;
    	for (i=0; i< 100; i++)
    	{
    		if( (i%3==0)&&(i%5==0)) printf("FizzBuzz");
    		else if(i%3==0)         printf ("Fizz");
    		else if(i%5==0)         printf ("Buzz");
    		else                    printf("%d",i);
    		printf("\n");
    	}
    }