What's the best way to put the "debugger;" statement inside the __doPostBack method or a way to step in the method?
__doPastBack is still javascript running in the browser.
While it's possible to debug javascript using visual studio and IE, if you want to set a break point you need to be able to open the javascript in visual studio. You can' do that for __doPostBack because it's generated by the compiler.
However, if you view the page in firefox you can use the firebug extension to set break points in the browser after the page loads.
In IE you can go to internet settings -> advanced -> and uncheck the "Disable Javascript debugging" boxes. This will allow you to put a breakpoint in the JS code.
You can manually attach to the iexplore.exe
process as Script debugger and set the breakpoint accordingly on generated page. Or you could break into debugger anytime by selecting the option from menu if you had enabled script debugging in advanced tab in Internet options
It really is just a lot easier to use FireBug inside of FireFox and you don't have to go through a lot of nonsense.
Another option for JS debugging and FireFox (which I use a lot) is to use venkman. You can get it as FF extension right here.