It assumes you have downloaded the iXhash.pm Perl module and placed it in /etc/spamassassin/.
Change path to iXhash.pm if necessary.
loadplugin Mail::SpamAssassin::Plugin::iXhash /etc/spamassassin/iXhash.pm
body GENERIC_IXHASH eval:ixhashtest('generic.ixhash.net')
describe GENERIC_IXHASH iXhash found @ generic.ixhash.net
tflags GENERIC_IXHASH net
# adjust score as you seem fit
score GENERIC_IXHASH 0.01
body NIXSPAM_IXHASH eval:ixhashtest('ix.dnsbl.manitu.net')
describe NIXSPAM_IXHASH iXhash found @ ix.dnsbl.manitu.net
tflags NIXSPAM_IXHASH net
# adjust score as you seem fit
score NIXSPAM_IXHASH 0.01
