0
Hello Naveen,
You need to try two or three solutions which are as follows:-
1. Add external file of Ajax by right clicking on your project and then clicking to Add Existing Item.
2. After Adding the external file drag and drop this file in the register section (above the head).
3. You need to Add Tools to the Ajax Toolkit. For this go to the Tools Window which will be on the left hand side of your project and from there go to the Ajax Tools options and then right click on these tools to Add new Tools. Add all the Tools of the Ajax by checking them all.
I hope now your problem will get solved.
Still if you are facing any problem then go through this Article:-
http://www.c-sharpcorner.com/UploadFile/cd7c2e/open-a-new-web-form-in-the-model-popup-of-Asp-Net-applicatio/
Here in the starting of this Article I had shown how to Add Ajax toolkit to your Application.
Thanks.