The first time you run the MailChimp plug-in for ACT! by clicking the icon on the toolbar or the menu item under the Tools menu, the program will prompt you to enter your MailChimp API Key. To do this, go to the Options>Preferences menu. You'll see a box to enter your API Key. If you don't know what it is, click the Locate your API Key next to that box.
Upon closing the window, the program will validate your api key.