The multi_manager is one of the most usefull entities in HL. And setting it up is very easy. An example:
Lets say you have a button. When you use this button, a light goes on. 2 seconds later, the ground shakes and some debris from the ceiling falls down. 4 seconds after that, an alarm sound is triggered.
Now how to set this up with a multi_manager?
Make the buttons first, and let it target start_seq_mm
Place a light entity and name it light1
Place an env_shake and name it shake1
Place an env_shooter and name it spawn_debris1
Place an ambient_generic and name it alarm_snd1
The properties of these are up to you.
Now place a multi_manager and name it start_seq_mm.
Now click the smartedit button. You'll see two more buttons: add and delete. Click add. Type light1 in the key field and 0 in the value field. Then click Ok.
Click add again. Now add shake1 as the new key and 2 as the new value. Click ok. Click add again. Now add spawn_debris1, 2. Click ok. Click add, alarm_snd1, 4. Click ok.
Now what happends is when you use the button, the multi_manager is triggeren and it will trigger all these entities you've just added. First the light. 2 seconds later, the shake and debris. 4 seconds later, an alarm is triggerd.
Thats all. Hope you understand it.