I suggest you to use Linq to Xml:
XDocument xdoc = XDocument.Load(@"C:\Users\ARNAUDR\Desktop\test.xml");
List<int> ids = xdoc.Root.Elements("PressRelease")
.Select(pr => (int)pr.Attribute("id"))
.ToList();
Note: your problem is that you always select first (single node) PressRelease node from root, and you are enumerating root nodes (which is always one) instead of enumerating PressRelease nodes. Here is fixed solution:
XmlNodeList pressReleases = doc.SelectNodes("/PressReleases/PressRelease");
List<int> IDsInDistantXML = new List<int>();
foreach (XmlNode pr in pressReleases)
IDsInDistantXML.Add(Convert.ToInt32(pr.Attributes["id"].Value));