If you get such kind of error Unknown Host dl-ssl.google.com while you try to download some Tools/Api's from Android SDK Manager, follow the below steps to resolve the issue.
See that your Android SDK Manager is open if not , go to Menu -> Windows -> Android SDK Manager.
Now if you are using Mac OS X goto Menu -> Android SDK Manager -> Preferences
If you are using Windows Operating System, goto Menu -> Tools -> Options
Now Check Force https:// … sources to be fetched using http:// …
This is because it maybe that you are not able to connect to the official Android repositories using HTTPS Protocol. Enabling this setting will force accessing it via HTTP Protocol.