Here is the contents of my slapt-getrc file which works great and has been working for a few weeks. Just use my sources and you should be good to go. Hope it helps.
Code:
# See /usr/doc/slapt-get-0.9.12/example.slapt-getrc
# for example source entries and configuration hints.
WORKINGDIR=/var/slapt-get
EXCLUDE=^kernel-.*,^alsa-.*,^glibc.*,.*-[0-9]dl$,^devs$,aaa_elflibs,x86_64,ruby,python,smb
SOURCE=ftp://slackware.cs.utah.edu/pub/slackware/slackware-12.0/
SOURCE=http://software.jaos.org/slackpacks/12.0/
SOURCE=http://darkstar.ist.utl.pt/slackware/addon/slacky/slackware-12.0
And the proper command syntax is slapt-get --update && slapt-get --upgrade
You can also overide the md5 problem with a --no-md5 flag on the upgrade command