Your Packing Key is the secret sentence that you should have chosen yourself when you signed up for Passpack.
When you signed up you should have been asked to provide three things:
1. a User ID
2. a Password
3. a Packing Key (a secret sentence, you chose on the screen that looks like the image below)
If your not able to remember your Packing Key, then you may simply try and sign up for a new free account - you can have as many as you like. You'll just need to choose a different User ID. Instructions for creating a Passpack account here.