Are you trying to set it for your own alliance?  

Lurker said:

There is no setting for a minimum level, Level 8 is the minimum level  for a player to join any alliance since that is when they can build an alliance center in the game.  The alliances that state a minimum level other than the default are stating a preference and actively remove those that do not meet it. 

So ooops what i said was wrong. There is a place to set minimum level, it must be somewhere only the marshal can see.

I was right about the level 8 stuff though  :-p

yes only the Marshall can set it.

It is located by selecting The Alliance tab and then at the bottom is a settings button.