Error on selecting XSD for schema

I tried to create an XML schema but when I try to import the XSD I get an error, as can be seen below.   I tried to first import said XSD and then try to import the problematic XSD again but that did not help. The XSD referenced resides in the same directory.  How can I fix this? [EDIT] The XSD belongs to the UBL format. The zip with the XSD’s can be downloaded from this location:  
1 answers

You could tools like this one: to check if the XSD is formatted right. If there are faults you might manually change them. If the XSD is freelly available you could edit your post so we can take a look or points us to the direction where the XSD can be found.