Hi Mazhar,
Thank you for the sample application. As I see, HTML Components (*.HTC) are used in it. This is the cause of the problem - HTML Components are not currently supported by TestComplete.
I tried to find a workaround by working with these *.HTC controls via Internet Explorer's DOM document using native methods and properties, but these tabs are not accessible even in this case. So, it looks like there is no way to work with these tab objects even from within the tested page itself. Please contact the application developers and ask them whether there is a way to work with the tabs in a script within the tested page. If there is a way to do this, most probably, it will be possible to adapt this way for TestComplete.