I had this problem. Since I already had the ADT address I could not follow the suggested fix. The reason why the update was not working in my case is that the ADT address was not checked in the list of "Available updates".
1) Go to eclipse > help > Install new software
2) Click on "Available Software site"
3) Check that you have the ADT address
4) If not add it following the Murtuza Kabul's steps
5) if yes check that the address is checked (checkbox on the left of the address)
I run the update after having launched Eclipse as administrator to be sure that it was not going to have problems accessing the system folders