Windows Marketplace for Mobile is now available for phones running Windows Mobile 6.0 or Windows Mobile 6.1.

Download Windows Marketplace for Mobile

Click this link to install the Marketplace software on your phone. After the file is installed, tap
Marketplace on your Start menu (or tap Start > Program files > Marketplace ) to finish the software installation.
This download is for all languages and for both touch and non-touch screen Windows phones.

If you have a Windows phone running Windows Mobile 6.5, you already have Marketplace! Just go to the Start screen and look for the Marketplace button.

source: mobile.microsoft.com