To use RD Web Access, your computer must be running at least Internet Explorer 6.0 and a version of Remote Desktop Connection ...

To use RD Web Access, your computer must be running at least Internet Explorer 6.0 and
  a version of Remote Desktop Connection (RDC) that supports at least Remote Desktop Protocol (RDP) 6.1.

To determine the RDP version that RDC supports, start Remote Desktop Connection, click the icon in the upper-left corner of the Remote Desktop Connection dialog box, and then click About. The RDP version that RDC supports is listed in the About Remote Desktop Connection dialog box.

RDC 6.1 supports Remote Desktop Protocol 6.1. RDC 6.1 is included with the following operating systems: