First off, make sure that the env_shake has a name.  That the multimanager has a value which targets the env_shake and has a delay.  Finaly, make sure you have a trigger to target your multimanager.

If all that is done and working, lets see what could go wrong with the env_shake entity itself.  Did you set an amplitude value high enough for you to recognise it?  Is the duration longer than just 1 second?  Is the jerk/rumble value a number between 0.1 and 255.0?  Do you have an effect radius value or the global shake flag ticked?

Thoses would be the things to look for, check, and if everything is right, then come again with more details as to what is doing on.  Does your mm target other entities?  Do thoses entities react to the mm?