1-If Adobe Photoshop is running then close it.
2-Open downloaded CSS3Ps.zpx file.
3-In Adobe Extension Manager click on Accept button.
4-When the installation process is done youll see the screen below. Now you can run Adobe Photoshop.
5-In Adobe Photoshop check Window - Extensions - CSS3Ps menu item.
6-Congratulations! Now you can select your layers and convert them to CSS.

1-Select layers that you want to convert
2-Select File - Scripts - Browse... menu item
3-Select CSS3Ps.jsx file in the folder where you have downloaded it
4-The script will run and open a browser window with the conversion result