When you start using Outlooks Google Apps synchronization, you are prompted to choose which Outlook-profile you want to use. If you don't want to answer this question again, you need to choose the default profile from Windows control panel.
1. Open Windows Control Panel.
2. Select Mail.
Easiest way to do this is to use the seacrh utility on the right corner.
3. Select 'Show Profiles...'
4. From the new window, select 'Always use this profile' and from the drop down menu select the account and click Confirm -> Ok.
5. Restart Outlook. You won't be prompted again during the start.
Note! In case you want to be prompted again for the account, you can change the setting back to 'Prompt for a profile to be used' from control panels mail section.