As everything in OpenERP, a server action is an object, and you want execute the method run

1. Create the server action you want to execute

2. Find its ID
You'll need the ID of the server action to pass as argument of your scheduled action. You can find it in several ways: select the action and look for id=<action_id> in the address bar, enable developer mode and select view_log in the developer options, find it in the database using psql or pgadmin...

3. Create the scheduled action
Use the following values:

• Object: ir.actions.server
• Method: run
• Arguments: ([<id_of_the_action],) e.g. ([596],)
  (cr and uid are passed automatically)