After you login the first time to SSRS you probably get the error message above. Well to fix this you can do the following.User ‘Domain\User’ does not have required permissions. Verify that sufficient permissions have been granted and Windows User Account Control (UAC) restrictions have been addressed.
1) Right click on Internet Explorer and run in Adminstrator mode
2) Type in the address localhost/reports
3) You will now be able to add the user to Site Settings and Folder Options.
Here is a reference link with screen captures.
http://www.soheib.com/technical-knowledge/sql-server-2012-reporting-services-uac-user-access-control/