Project wouldn't run after importing to Eclipse

With the following error message:
"The selection cannot be launched, and there are no recent launches"

I got it for my Android project, but it's not limited to android.
Since I'm newbie to Eclipse I started to googling but I couldn't find a solution. Playing with the UI got me to the right place:

In the main menu: "Run" -> "Run configurations..."
Double click the project type you want (in my case "Android Application") & select the project you want to run.