The routing is looking for an Index Action on your controller if the action was not specified in the route or by entering into the url. Rename your GetTip function to Index.
If that is not acceptable, you can add a route similar to the following in lieu of your current route.
routes.MapHttpRoute("TipOfTheDay", "api/tips", new { Controller = "TipOfTheDay" Action = "GetTip" });
I wouldn't recommend this route, however, since it will try to use GetTip as the default action every time one is not specified.
Here is a good resource for routing in a web forms application: http://msdn.microsoft.com/en-us/library/dd329551.ASPX