I have an Alesis SR-16 drum machine, which for the purpose of recording with RW, it's the same thing. I ran one of the 1/4" outs into my toneport and used one of the Mic Preamp models to record with.
But here's the kicker. Since RW is not yet midi-capable, you have to start recording and start your drum pattern on the Alesis at EXACTLY the right time, otherwise the beat will not loop properly in RW. While it can be done, the process is so laborious that to me it's not really worth it.
If your SR-18 and my SR-16 have the same ins and outs (and I think they do), you should be able to run a 1/4" cable from your MAIN RIGHT output (that should be a mono signal). You could experiment with the different outputs to see what works best. But I'd advise programming your entire song on the Alesis then recording it on the computer using a linear recording app of some sort, as much as I hate to recommend something other than RW
