Since the release of Windows 7 SP1, people who installed the Service Pack before installing the RSAT package weren’t able to do so (although if you installed RSAT before SP1 you were fine).

Microsoft have resolved this with the release of Remote Server Administration Tools for Windows 7 with Service Pack 1 (SP1): http://www.microsoft.com/downloads/en/details.aspx?FamilyID=7d2f6ad7-656b-4313-a005-4e344e43997d