Did you get any errors when installing?
Here's what I suggest doing:
- Rename /admincp/ranks_orig.php to /admincp/ranks_xxx.php
- Uninstall this product.
- Delete /admincp/ranks.php
- Rename /admincp/ranks_xxx.php to /admincp/ranks.php
- Upload a copy of "functions_ranks.php" to your "includes" folder.
- Install the product again.
- Verifiy the 4 files I asked about before are all present. If so, see if the rank ordering function works.