The current DMux8Way.cmp file in the Nand2tetris Software Suite Version 2.7 download is blank.
The file should (and has in previous downloads) contain:
| in  |  sel  |  a  |  b  |  c  |  d  |  e  |  f  |  g  |  h  |
|  0  |  000  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  0  |  001  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  0  |  010  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  0  |  011  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  0  |  100  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  0  |  101  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  0  |  110  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  0  |  111  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  1  |  000  |  1  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
|  1  |  001  |  0  |  1  |  0  |  0  |  0  |  0  |  0  |  0  |
|  1  |  010  |  0  |  0  |  1  |  0  |  0  |  0  |  0  |  0  |
|  1  |  011  |  0  |  0  |  0  |  1  |  0  |  0  |  0  |  0  |
|  1  |  100  |  0  |  0  |  0  |  0  |  1  |  0  |  0  |  0  |
|  1  |  101  |  0  |  0  |  0  |  0  |  0  |  1  |  0  |  0  |
|  1  |  110  |  0  |  0  |  0  |  0  |  0  |  0  |  1  |  0  |
|  1  |  111  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  1  |