Hi Michael,
This is already a known issue which has been fixed internally in our latest source code and this fix will be available in our next release.
Defect #729 - Unable to switch tab pages in the TabControlAdv at design time.
As for now, if you set FocusOnTabClick property to false, you could be able to click on the tabs.
Please let me know if you have any questions. Thanks for using Syncfusion products.
Regards,
Mano