Can you check if this file is present in the Sql Server Installation location
<Microsoft SQL Server Installation>\90\DTS\Tasks\Microsoft.SqlServer.MaintenancePlanTasks.dll.
If the file is not present it may not be able to load the Maintenance Tasks, For Control flow tasks related entities all the dlls are copied to this folder(can be 3rd party components also).
If the Microsoft related dlls are not present then there might have been a problem with the installation.
