I am from Guwahati,Assam. I couldnot recharge my 3G data plan at portal.bsnl.in. This option is not available at all circles. Now I have found an alternative to it-Rechargeguru. Her you can do online recharge of BSNL 3G data plan if you failed at BSNL site. Some extra charges will be levied.
2.Select BSNL recharge.
3.Select BSNL On Demand or Custom Made Online Recharge
4. Press the "Add to Cart" blue button to go to next page.
5.Press Order Now button.
6. At "Special Instructions or Comments About Your Order" entered following -"Please recharge my 3G Data Plan with with Rs.275 data plan" Then press "Continue Checkout".
7. At payment page select this option E-Billing Solution v2.5 Indian Rupee (INR) Payment Gateway | ITZ Card payment for payment.Some extra charges will be levied but its better than going to BSNL franchisee or office.
8. After payment my data plan was recharged within 10 minutes.My advise contact the site ad before doing any 3G Data recharge, so that they can provide you information whether online recharge for your area is available or not.