Did you re-install it AFTER you installed Vegas Pro 8.0? If Vegas Pro 8.0 wasn't installed when you installed StillMotion it would have not been able to add itself to the Vegas Pro 8 menus (which would not have been there)
Also did you download the latest version of StillMotion that works with Vegas Pro 8? The version that worked with Vegas 7 will not work with Vegas Pro 8 because Sony chances the script engine with every release of Vegas. Simply download the latest version from the VASST web site and run the installer.
Also be sure you are using Vegas pro 8.0b Build 217. Previous versions of Vegas Pro 8 did not handle script resolution correctly and causes failures when loading.