You have to register your API Key for the Redmine account. If you do not have an account on Redmine, you can skip this (but please confirm it with your project manager).
Menu -> Account Link
RedSync displays a dialog to input the API Key. You can get the API Key from your Redmine account.
Refer to How To Get Redmine API Key.
Validate your API Key by clicking the Validate button. RedSync shows the user name associated with the API Key.