A dataset is like a in-memory database. This can contain one or more tables. To get the retrieved value from your excel file, you should go inside its table and access the cell there.
Since your select query there returns only one cell from the excel file, you can probably access the value from the dataset this way.
Label1.Text = ds.Tables[0].Rows[0][0].ToString();
UPDATE:
It seems that it's treating the value as a header column. I tried your code and made some modifications on the connection string, just added HDR=NO. You may try the code below on your end.
string connStr = String.Format(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source={0};Extended Properties=""Excel 8.0;HDR=NO;""", file);