The easiest way: You should sign in in your current WU account.
Then click the link https://www.wunderground.com/member/api-keys
This is the last mail:
Dear PWS uploaders, thank you for your continued loyalty and patience!The gateway for the new API keys for PWS uploaders is open.
Cut and paste this URL into your browser: https://www.wunderground.com/member/api-keys
If you are not logged in, you will be asked to. The system will also verify that you have a PWS uploading to the system.
Wim