For simplicity's sake, put one of the azw files in the same folder as mobi2mobi.exe (not the gui program, the folder where all the mobiperl executables are). Then open a command prompt and change (cd) to that folder.
...and hitting Enter will spit out everything that is known about the file. At the top of that list will be "Database name:" The title of the book (or part of the title) should appear there. If there is nothing there, then the file won't work with Mobi2Mobi-Gui. In that case:
mobi2mobi --databasename "Some_Name 32_characters or less" --outfile "New ebook file name so I don't screw up the original one.azw" "bookname.azw"
...and hitting Enter will create a new azw file (with the name you gave for the --outfile parameter) that should work with Mobi2Mobi-Gui.