function searching in an array of structures - stop condition unknown

The problems are with your findword function, lets go through all the lines

struct words *ptr; 

This is not what you ment to do. The typedef you used in defining the structure allows you to not need to write struct anymore. This is why you're getting the error: reverse.c:73: error: increment of pointer to unknown structure. What you want is just:

words *ptr;    

Next, the loop:

for(ptr=data; //This is fine, you're assigning your local ptr to the global data. I assume that's something valid

ptr != NULL; //That could OK too... we can loop while ptr is not NULL
ptr++)       //This line makes no sense... 

You may want to look up how for loops work again, the point is you're incrementing something until it hits a condition. ptr++ will move where you're pointing too, so you'll no longer be pointing to your structure.

I need to see your main() function to understand what you're trying to accomplish, but based on the prototype you have to follow, I think the easiest solution would be something like:

void main()
{
    // init your vars
    bool more_words_to_add = true;
    words *ptr = NULL;
    int i;

    // populate your array of words
    while(more_words_to_add) {
        for(i = 0; i<max; i++) {
          if(ptr = findword("word"))  //if we find the word
            ptr->occ++;  //increment the number of times we found it
          else {
            //I don't know what you want to do here, it's not clear what your goal is.
            //Add the new word to your array of words and set occ to 1,
            //then increment max because there's one more word in your array?
          }
        }
        //get the next word to fine, or else set more_words_to_add = false to break
    }
}

If this type of solution is what you're looking to do, then you can adjust your findwords function to be very simple:

struct words *findword(char *word)
{
    words *ptr = data;
    if (strcmp(word, ptr->word) == 0)
        return ptr;
    return NULL;
}  

EDIT: For your new error I suspect the problem is with your memory allocation, see this short example of using your structure:

words *findword(char *word)
{
    words *ptr = data;
    if(strcmp(word, ptr->word) == 0)
      return ptr;
    return NULL;
}

int main(){
    words *ptr;

    data = realloc(data, sizeof(words));
    data->word = "hello";                //DO NOT SKIP THESE LINES
    data->occ = 0;                       //DO NOT SKIP THESE LINES

    if(ptr = findword("hello")) {
      ptr->occ++;
      printf("I found %d %s's\n",ptr->occ, ptr->word);
    }
} 

mike@linux-4puc:~> ./a.out 
I found 1 hello's

You can see here that you need to alloc some memory for the global structure then you can store data in it and pass pointers to it.

EDIT 2:

Your main() code does this:

if((ptr = findword(word)))
{
     //do stuff
}
else
  ptr->occ++;

That's not going to work because if findword() fails it returns NULL, so in the if check ptr is set to NULL, then in the else you're trying to deference NULL. If (and keep in mind I'm not really reading your logic so this is up to you) you really want to increment ptr->occ if a word is not found then you want this instead:

if(findword(word))
{
     ptr = findword(word);
     //do stuff
}
else
  ptr->occ++; //increments the current ptr's occ, no new ptr was assigned.