This software comes with Simatic Step 7 Professional Edition. You can also install it separately as a 14 day demo from an S7 Pro CD even if you don't have the Pro license.
I'm pretty sure there is not a Siemens site that you can download the software from. However, ask your local Siemens distributor and they will probably help you get the install software. It's called S7-PLCSIM.
Let me know if you are having trouble getting it.