wouldnt it be wise just to open the file with Open dialog?
Of course you can browse workbooks collection (contains all open wbks in this instance of excel).
something like
dim wbk as excel.workbook
for each wbk in application.workbooks
  if wbk.name <> thisworkbook.name then
  ' populate some control in Userform like list.add or something
   ' or check some characteristic sign that "this is the file" an if "it is -> exit for
  end if
next wbk