select="document(concat('Z:\DicionarioXml\',$MasterTable,'.xml'))

                          ^^^^^
Note: the argument passed to the document() function should be an
URL, not a Windows path name. Just in case.

... /Estrutura/.../Campo[Nome=@Nome]/Visivel"/>

                      ^^^^^^^^^^^^^^^^
You are selecting Campo elements which have a Nome child element
which has the same value as the Nome atttribute on *the Campo
element*. You probably either want to use current()/@Nome instead
or get the required value into a variable and use it.