I am following these two articles to create a workflow that will create Accoutname for a user using firstname and lastname when user is created in FIM portal.

https://msdn.microsoft.com/en-us/library/windows/desktop/ff859524(v=vs.100).aspx and 

http://www.fimspecialist.com/fim-portal/custom-workflow-examples/generate-unique-attribute-workflow-using-enumerate-resources-activity/

I was able to successfully build the solution and created the workflow in FIM attached to an MPR-> administration:administrator and read and update users.

When i create a user . two requests are generated.

One is post processing error . Details are 

<RequestStatusDetail xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" 

DetailLevel="Information" EntryTime="2015-07-

30T13:01:04.7015693Z">Microsoft.ResourceManagement.WebServices.Exceptions.PermissionDeniedException: ManagementPolicyRule
   at Microsoft.ResourceManagement.Workflow.Hosting.RequestWorkItemProcessor.DispatchRequest[TResponseType](RequestType request, 

Boolean applyAuthorizationPolicy)
   at Microsoft.ResourceManagement.Workflow.Hosting.RequestWorkItemProcessor.ProcessPutWorkItem(UpdateRequestWorkItem 

updateWorkItem)
   at Microsoft.ResourceManagement.Workflow.Hosting.RequestWorkItemProcessor.ProcessWorkItem(WorkItem 

workItem)</RequestStatusDetail>
------------------------------------------------------------

However user is created. and then another request is generated to update account name by the user itself not administrator

For second request , Denied.

Stack Trace is as following

Microsoft.ResourceManagement.WebServices.Exceptions.PermissionDeniedException: ManagementPolicyRule ---> System.Data.SqlClient.SqlException: Reraised Error 50000, Level 16, State 1, Procedure DoEvaluateRequestInner, Line 1319, Message: Permission denied: <ai><Name>AccountName</Name></ai>
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)
   at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)
   at System.Data.SqlClient.SqlDataReader.ConsumeMetaData()
   at System.Data.SqlClient.SqlDataReader.get_MetaData()
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader()
   at Microsoft.ResourceManagement.Data.DataAccess.EvaluateRequest(RequestType request, RequestEvaluationOptions options)
   --- End of inner exception stack trace ---

any help is appreciated.

Looks like a simple error of permissions.  Check the request, who is making the request.

If it is the end user, then you need to modify the MPR "users can modify attributes or their own and select accountname as well"