You need to register your command in Sitecore
configuration. Either create your own MyCommands.config
file in App_Config/Include
with content:
<configuration>
<command name="webedit:addlink" type="My.Assembly.Namespace,My.Assembly"/>
</configuration>
or update one of the existing files with the command definition.
EDIT IN ANSWER TO YOUR COMMENT:
The reason why your code doesn't work is that you're trying to redirect the response of the POST call which is requested while button is clicked. What you need to do is to register a javascript that will redirect the parent window after the response from Sitecore comes back, like TwentyGotoTen wrote in his post below.
public override void Execute(CommandContext context)
{
Sitecore.Web.UI.Sheer.SheerResponse
.Eval("window.top.location.href='http://google.com';");
}