According the PremierePro SDK the installer searches the registry for the installation path:
- HKLM\SOFTWARE\Adobe\Premiere Pro\CurrentVersion\Plug-InsDir
- HKLM\SOFTWARE\Adobe\Premiere Pro\[Latest Version]\CommonPluginInstallPath
- c:\
Just select the plugins folder manually. i.e.: C:\Program Files\Adobe\Common\Plug-ins\7.0\MediaCore