Can you give some more details please?
Are you getting an error message? if so what is the error?
make sure you are following the proper download / install procedures
When you click the download link to download Design Extender, if you get the option to save the File Or Open it, make sure you select the option to save it.
once the download completes, open the extension manager, and click the install button, browse to the downloaded MXP file and select it to start the install.