You can just follow those instructions on the /setup page again to re-add your key and secret, just like you did the first time (you can skip creating the key in bitbucket, reusing the one that's already there). It will use your same credentials for Bitbucket, and then when you log into the editor, it will load your My-Pi-Projects repository from bitbucket automatically for you.
The reason you need to do this step again is that we store the consumer key and secret within a database on your Pi SD card, so the new SD card doesn't have this info anymore.
The consumer key and secret are a component of the OAuth protocol. This allows you to associate your WebIDE with your Bitbucket account, in order to give it access to the repositories held within.
Let me know if this doesn't work!