To download simBio, go to http://sourceforge.net/projects/simbio/ and click on the "Download simBio" button.
Next, scroll down to the "File Releases" list and click on "simBio-*.bin.zip" to download the latest version of the simBio binary package.
The download should begin automatically. If it doesn't, please click on the words "direct link" on the web page.
All previous releases of simBio can be found in the http://sourceforge.net/projects/simbio/.
After downloading the zipped simBio package, place it in the directory of your choice, then right-click on it and select "Extract All" from the popup menu. Follow the instructions in the Extraction Wizard to extract simBio to the folder of your choice.