Sorry, Frank, but what else can I tell you?
Here's a default Slackware-12.0 system, which shows the kernel has that module supported:
anna@peter:~$ cat /etc/slackware-version
anna@peter:~$ uname -a
Linux peter 18.104.22.168-smp #2 SMP Tue Jun 19 14:58:11 CDT 2007 i686 AMD Athlon(TM) XP 2000+ AuthenticAMD GNU/Linux
anna@peter:~$ cat /usr/src/linux/.config | grep -i advansys
I really don't know what else to say, or why it would not work for you. Here's from the Slackware-11.0 CD1, okay?
root@silas:~# mount -oloop /serverhome/slackware-11.0-iso/slackware-11.0-install-d1.iso /mnt/hd/
mingdao@silas:/serverhome$ less /mnt/hd/kernels/scsi2.s/config | grep -i advansys
And from my laptop's old Slackware-11.0 system, which I have not yet removed:
mingdao@titus:~$ cat /etc/slackware-version
mingdao@titus:~$ uname -a
Linux titus 22.214.171.124 #1 Fri Sep 1 01:48:52 CDT 2006 i686 pentium4 i386 GNU/Linux
mingdao@titus:~$ cat /usr/src/linux-126.96.36.199/.config | grep -i advansys
So perhaps the fact that it's built-in to the Slackware-11.0 scsi2.s kernel, and a module with the Slackware-12.0 boot kernels, is the reason it doesn't detect your SCSI card.
There are maybe several ways to boot with that Slackware-11.0 kernel, then install Slackware-12.0. You can Google for them.
Do you have space to make a repository of Slackware-12.0? If so, you can copy all the files from Slackware-12.0 CD1 and CD2 into a directory, then boot with the Slackware-11.0 CD but install from the Slackware-12.0 directory, rather than the Slackware-11.0 CD.
I'm sure there are instructions out there to do it, if you'll just look. Please accept my apologies for not doing more research on this for you.