short:
$ man patch
long (and possibly more useful :) :
$ cd /openacs/top/level/dir
$ patch <patch.file
/openacs/top/level/dir is where your OpenACS is installed, something like /web if you follow installation howto
patch.file is the patch file you obtained somewhere
As noted on SDM page, all patches are supposed to be generated from the top level directory of OpenACS.
