Music Composers Unite!
I forget which version you have. Is it 6? As you recall, Sibelius will only open certain types of xml files. Try using the Open function. Then set the file type to All or maybe All Music Files.
I have v.7 and have tried what you suggest already - I've posted the same question on the Sibelius Forum (yesterday) and still await a response.
But thanks for trying.
They will probably ask you to post the file so they can try it. You could sen it to me if you like. Sometimes MuseScore has better luck with xml that Sibelius.