One of two problems, either the Label findcontrol is not pulling a result or the Table is in fact not pulling a result. Easy way to figure that out is to grab the table as a string and the label as a label.
Label displayNameLabel = DetailsView1.FindControl("lblDisplayName") as Label;
if (displayNameLabel == null)
string testResult = ds.Tables.Rows.ToString();
Then it's just a matter of correcting the individual error. This is a very widely requested issue though and the finding of controls can be tricky. It must be a template, and it must of course be a label, and if it's in DetailsView you can easily set up binding from the designer instead of manually in code behind... Google DetailsView binding for more help or look at this example Using TemplateFields in the DetailsView Control