Here's what you need to do:
1) Create a new Silverlight Windows Phone Application
2) In your MainPage.cs and add a using directive to the Microsoft.Phone.Tasks namespace.
3) Add the following in the constructor of MainPage.cs after calling the InitializeComponent() method:
var task = new MarketplaceHubTask();task.ContentType = MarketplaceContentType.Applications;task.Show();
4) Launch the application and you should be good to go.
Here's how the Marketplace looks like on the emulator:
Click here to see a list of supported launchers and choosers for Windows Phone 7. You can read more about Launchers and Choosers here.