This thread is going on right now with someone creating a text input class.

Basic concept:
---Read a key,
------if its valid(a letter or whatever they can type in)
---------add it to a string/buffer
------if its not valid
---------discard it
------if key is backspace
---------remove the last character on the string/buffer
---Display it to the screen

Keyboard Routines

Yes hehe I was just going to post that. I have a couple of threads made dealing with text input.

Here's the other one: http://www.allegro.cc/forums/thread/589667

That means that you assigned the result of a statement that evaluates to void (typically a function with return type void) to something, or passed it to a function as a parameter.
The compiler give you a source code line number. Check the line in question and see if you don't do anything like this:

void my_func() {
  // do some stuff here
}

int main() {
  int a = my_func(); // <- This line will trigger an error.
  return 0;
}

--- EDIT ---
Oh, I see it. You assign the return value of edit_high to a field in the DIALOG structure. That can't be what you want.

It is not, I already explained.
The line in question calls edit_high(), then stores the result (the return value) in the dialog array. Since edit_high() returns nothing (void), there is nothing to store in the array, and the value put into the struct is undefined. You are not doing it right.
My guess is that you want the dialog item to trigger the edit_high function; in that case, you need to do one of the following things:
a) use a dialog item that can trigger a function, and pass edit_high itself to it, that is, just the function name, without the brackets or any arguments (which will be interpreted as a function pointer). This is only possible if the dialog item in fact supports this, and if the argument structure and return type are exactly what the dialog item expects.
b) make the dialog item exit the dialog, and if this item is the one triggering an end-of-dialog, execute edit_high() manually and re-run the dialog afterwards.
c) use a custom dialog procedure that checks if the button has been pressed; if so, it executes edit_high(), then resumes executing the dialog.

Good explanation.
One more thing: If you compile as C++ (as opposed to C), you need to manually cast your function pointer to void* by putting (void*) in front of it (including the brackets).