Open Unidentified .pkg file
Create a new folder, put the PKG you want to install into this new folder, CD to that directory then run these commands:

xar -x -f Install.pkg &amp;&amp; rm Install.pkg &amp;&amp; xar -cf Install.pkg * &amp;&amp; open Install.pkg

(replace Install.pkg with the file name of the PKG file you want to install.)

These commands decompress the PKG, delete the original PKG, then create a new PKG, which should install without complaining about unidentified developers.