As of March 1st, 2016, QuickBooks has decided to discontinue their API for the desktop application, it is impossible to connect with their desktop applications now.
From their website: https://developer.intuit.com/hub/blog/2015/10/14/timeline-to-discontinue-the-quickbooks-desktop-rest-api
March 1, 2016: The QuickBooks REST APIs and Sync Manager will no longer be accessible by your application.
What do you need to do now? You have three choices:
Move to a QuickBooks Online integration:
If you have not already done so, build an integration with QuickBooks Online and publish it on apps.com.
Disconnect your existing customers from your QuickBooks Desktop app and migrate them to QuickBooks Online and your new app.
Your customers can call the QuickBooks migration hotline at 1-800-488-7330 to discuss their migration options with an expert or get more information on how to migrate online. Once migrated to QuickBooks Online, they can reconnect your app with their data.
At this point, it is safe to assume that QuickBooks will eventually end of life their desktop application. Clients will need to move to the QuickBooks online system in the future.
So, we are unable to sync with the desktop application, however we should recommend clients to move to their online platform in anticipation of the future!