Opening a managed users worklist from code is pretty simple. Please read this documentation.
However, there is a flag that needs to be set to allow for managers to open managed worklists. This could be found in the old K2 Workspace, however in K2 Five without access to the old Workspace it does not exist.
When you try to call a managed user's worklist, you get a [MANAGER USER] is not authorized to open this worklist item.
Use the following SQL query to enable opening managed users worklists:
exec [Server].[mServerSettingsSave] @XMLDoc = '<Root><ServerSetting Name="ManagedUsers" Value="True"/></Root>'
Restart the K2 Server after running this query for it to take effect. Please create a backup of the Database before running the query.
It is highly recommended to make a backup of the K2 Database before making any changes. Please make a DB backup before running this query. If you do not comfortable to do so - please contact K2 Support.
Also see: http://help.k2.com/onlinehelp/k2blackpearl/devref/current/default.htm#Database_Reference.html