Trouble with opening an XML (that is possibly missing a stylesheet)

Hi all,
unfortunately I've arrived at a point where i cannot help myself anymore.
I am trying to open an xml, but I get an error message saying that the file does not have a schema. After the file opens but the format is not right and the data cannot be used.
I dont have the exact error message because I use german excel, but it is something along the lines of " the xml source does not refer to a schema. Excel will generate a schema based on the xml source".

Thanks a lot for any help you can give me.