When attempting to create a maintenance plan in SQL Server Studio, or you are attempting to view the SQL Agent settings, you may get an error about Agent XPs not being enabled. Agent XPs (Agent eXtended Procedures) is a requirement for SQL Agent to be configured through SQL Server Studio, regardless of whether the SQL Agent is running or not. This also includes the creation and modification of maintenance plans.
To enable Agent XPs, run this query on the instance (either through SQL Server Studio or osql):
sp_configure 'show advanced options', 1; GO RECONFIGURE; GO sp_configure 'Agent XPs', 1; GO RECONFIGURE GO
The change is effective immediately (does not require a server stop/start). Note that you'll need to have sysadmin privileges on the instance.