If you have not added ADT then do the Following steps.
Help -> Add New Software -> Add.
In Name
option write ADT
And if you have installed ADT in your Computer then add its path in Location
like
jar:file:/D:/ADT-16.0.1.zip!/
And then ADT will install.
And if there is a problem try to remove that adt and install again.
1) remove the ADT repo (Window->Preferences->Install/Updates->Available Software Sites)
2) add Indigo repo ("http://download.eclipse.org/releases/indigo")
3) run updates
4) add ADT repo again and install
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