To install Spiro, do the following:
1. Download the package for your system.
2. Upzip the package, and get a folder named Spiro.
3. Move the whole Spiro folder into Illustrator's Plug-ins folder.
The following picture shows the directory structure on Windows, and the Mac is the same. If the tool doesn't show up, please make sure that you have placed the Spiro folder properly.
Without a license, the software runs in demo mode, and can not draw splines with more than 6 nodes. You can buy a license and install it to enable full function of the software.
Read the page How to Install a License , and following the directions to install your license.