You need to specify the correct url when calling the action. In your struts.xml file you would have created an action entry with a name. Let's say that name is VisitorCounterAction. Note that this is not the name of the Action class but the name you use in struts.xml.
Also you would have configured struts to recognize a particular extension. By default it is .do.
To request the action you will then use: VisitorCounterAction.do.
So try:
xmlhttp.open("GET","VisitorCounterAction.do",false);
Note also the double quotes around the URL as this is a String in the xmlHttp object's open() method.