Start Eclipse, then select Help > Install New Software.
Click Add, in the top-right corner.
In the Add Repository dialog that appears, enter “ADT Plugin” for the Name and the following URL for the Location:
Note: The Android Developer Tools update site requires a secure connection. Make sure the update site URL you enter starts with HTTPS.
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.
If you get a security warning saying that the authenticity or validity of the software can’t be established, click OK.
When the installation completes, restart Eclipse.