The functionality of managing permissions via SQL Server Management Studio had been removed in Reporting Services 2008.

A new sample, RSPermissions which demonstrates how to write an application which manages Reporting Services permissions via the Reporting Services Web Service API is currently available http://msftrsprodsamples.codeplex.com/Wiki/View.aspx?title=SS2008%21RSPermissions%20Sample%20Application.

This had been included as a part of SQL Server 2008 (Katmai) Product Samples Refresh 3.