check this link: https://developer.android.com/sdk/installing/installing-adt.html#Troubleshooting
- download adt from https://dl.google.com/android/ADT-22.6.2.zip
- Start Eclipse, then select Help > Install New Software.
- Click Add, in the top-right corner.
- In the Add Repository dialog, click Archive.
- Select the downloaded ADT-22.6.2.zip file and click OK.
- Enter "ADT Plugin" for the name and click OK.
- In the Available Software dialog, select the checkbox next to Developer Tools and click Next.
- In the next window, you'll see a list of the tools to be downloaded. Click Next.
- Read and accept the license agreements, then click Finish.
- When the installation completes, restart Eclipse.