If you're using SI 2012 (32 bit) it won't show up (bug).
But it works in SI 2011 / 2011SAP (32 bit). Try to use it in one of those, or perhaps even 2010.
I think it works on 2012 (64 bit), but I'm not sure. I use a 32 bit system, and got the same problem as you.
However, I use Arnold with SI 2011 SAP without any problems.
BTW, don't forget to check the "skip license..." thing in render options, before you try to render anything.