How to get FB SDK accesToken for page ?

Hello, I'm testing the facebook SDK, so far I can LogIn as an user and see me personal infos, now I'd like to access to a page, for that I need an acces to page tokens, Once I'm logged, i'm trying like this:

 perms.Add ("public_profile");
         perms.Add ("manage_pages");
         FB.LogInWithReadPermissions(perms, AuthCallback);

 FB.API ("/XXXXXXX?fields=access_token", HttpMethod.GET, getToken);//where XXXX stands for my pageID
     void getToken(IResult result){
             print ("res token"+result.ResultDictionary ["access_token"]);
         }

problem : I get a "Graph API error: 400 Bad Request", as if the access_token is not an available info from my API request.

What is a good way to get that token access?