jQuery + Foursquare API, returns me a "null" Object from JSON

jQuery + Foursquare API, returns me a "null" Object from JSON

Hello folks!! 
(excuse my english if I'm wrong) 


I use the advantage of JSON by the capability to be 
Crossdomain, I tried several times to parse and get data from the Foursquare's API, but, at time when I try to show data, the script simply does nothing, and, with some alerts, I figured out that the trouble appears at the moment to get the '"json" result, the reason: give's me a NULL, I mean, brings nothing with it. BTW I have to do it only with javascript, for this, I use JQuery to get it better and easier, but, I still don't get it. The 2 ways that I tried to do this are this: 

//With Ajax 
    $.ajax({ 
      url: 'http://api.foursquare.com/v1/tips.json?geolat=45.5&geolong=-73.6666667&limit=50', 
      //data:'geolat=45.5&geolong=-73.6666667&limit=50',
      async: false, 
      dataType: 'json', 
      type: 'get', 
      success : function(tips) { 
                alert(tips+' Show tips content'); 
                var str=''; 
                for (var i=0;i < tips.groups[0].tips.length;i++) { 
                  var cur = tips.groups[0].tips[i]; 
                  str = str+'<p><img src="'+cur.user.photo+' "style="width:32px;height:32px;" />'+cur.text+' by '+cur.user.firstname+' '+cur.user.lastname+' on <a href="http://foursquare.com/venue/'+cur.venue.id+'">'+cur.venue.name+'</a></p>'; 
                } 
                $('#items').append(str); 
              } 
    }); 

















//With getJSON 
    $.getJSON(' http://api.foursquare.com/v1/tips.json', 'geolat=45.5&geolong=-73.6666667&limit=50', function(tips, didit) { 
       alert(tips+' Show tips content and succed '+didit ); 
       var str=''; 
       for (var i=0;i < tips.groups[0].tips.length;i++) { 
        var cur = tips.groups[0].tips[i]; 
        str = str+'<p><img src="'+cur.user.photo+'" style="width:  32px;height:32px;" />'+cur.text+' by '+cur.user.firstname +'  '+cur.user.lastname+' on <a href=" http://foursquare.com/venue/'+cur.venue.id+'">'+cur.venue.name+'</a></p>'; 
       } 
       $('#items').append(str); 
    }, 'json'); 
    }); 











I hope you can help me