Heh.
Well you need to use a game_counter. Get the first wave of monsters to trigger the game counter when they die, this is done in the "trigger target" and "trigger condition" fields.
Call the game counter "counter01" (done in the "name" field) and make it target "wave02" done in the target field. This means that in the "trigger target" field for the first wave of monsters, you put in "counter01". In the game counter, in the "limit value" field, put the number of guys you want to die before the next wave eg: "3" [or] "5"....
Now create a monster_maker, call it "wave02" and enter in the details! When your entering in the "monsters type" you need to use full name, not "zombie" or "headcrab", "monster_zombie".....
Create as many monster_makers as you need under the same name "wave02". This means that once the certain amount of monsters die, the game counter will trigger everything called "wave02", so all the monsters will spawn.