You can only use Microsoft Marketplace with Mango

From now on, we can only make full use of the Redmond store with WP 7.5.

Microsoft has also officially announced that from now on we will only be able to use all the features of the Marketplace from devices running Windows Phone 7.5. In practice, this means that using a version older than 7.5, we can only browse applications, but it is no longer possible to install a new one or upgrade an existing one. Anyone who may not have upgraded the system is now recommended to get the new version, as this is the only way to use the smartphone to its full potential.

Source: gsmarena.com