With a Pro Account, you can do a Historic Sync by editing your Withings task and selecting Historic Sync from the bottom. This will allow you to select the dates you want to copy into FitnessSyncer and the operation will occur in the background. It could take a while to complete and it is recommended that you not change the settings while it is in progress.
When you authenticate with Withings, it will ask you wish user to share. If this is not working, please contact us.