Well I don't know why, but your code does not show any dialog, but it's pretty simple, your value of count is remaining 1 because. you are declaring it as 0 within the function, so whenever you press the button it declares it 0 and then increases it by 1, then again you press the button and it declares it as 0 and increases it by one, so just declare out of the function it's self, below the num variable.
I made this code for you hope it helps:
Code:
<html>
<head>
<SCRIPT LANGUAGE="JavaScript">
var num = Math.floor(Math.random() * 101);
var count = 0;
function guessnum()
{
var guess = document.forms['form1'].num.value;
if (guess == num)
{
count+=1;
alert("Great you Guessed! took you" + count);
}
if (guess < num)
{
count+=1;
alert("No your number is too low! Count " + count);
}
if (guess > num)
{
count+=1;
alert("No your number is too high Count " + count);
}
}
</SCRIPT>
<title>Guess the Number</title>
</head>
<body>
<form name="form1">
Enter a number <input type="text" size=5 maxlength=3 name="num"> <input type="button" onClick="guessnum();" value="Enter">
</form>
<div id="counter">
</div>
</body>
</html>