Cannot Access Content From Json File

I am trying to login in a website via HTTP post and then via HTTP get download a json file (.do file with json content). This json file url contains a group of parameters that I already sent on the url. Although, I can successfully login I cannot access the content from the json file - I'm downloading the html code of the site - i have already tried for an image or a css and it works.The code goes like this:

Code:
HttpClient client = new DefaultHttpClient();
HttpPost req_post = new HttpPost(url_post);
List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
[code]...