how can I turn on swap? swapon didn't work...

Hi, I am running Suse 9.2 on a machine with 2 gigs of ram. I try using
matla to open a data file, and it gives me "out of memory" errors. In a
shell, free tells me swap usage is 0.

I've tried /sbin/swapon -a . But I continue to have the same behavior.
I can't seem to get swap on.

I have a separate 4 gig partition devoted to swap. I did the standard
install for Suse, which on other machines left swap working fine, but
for some reason on this machine I'm stuck with no swap.

Thanks in advance for any advice....