You can find SpamAssassin's local.cf file in one of two places depending on your server version.

The local.cf file on a 32-bit server can be found here: C:\Program Files\Smartertools\Smartermail\Service\SAData\local.cfg

The local.cf file on a 64-bit server can be found here: C:\Program Files (x86)\Smartertools\Smartermail\Service\SAData\local.cfg

Please note that if you have Smartermail installed to a different drive letter other than C, you would follow the same paths above, but according to the drive letter you are using.