1.Create a standard sized room and place in there a player start and a light.

2.Just in front of the info_player_start make a brush and attach the trigger texture to it then you can make it a trigger_multiple or trigger_once, give it these values as well

"target" "alarm"
"spawnflags" "1"
"classname" "trigger_multiple"

3.Now place a monster in to the level (we'll use a monster_soldier), give him this bit of info

"targetname" "ala1"
"target" "ala2"

4.Now somewhere in the level make a brush that'll be the button and just in front of that make another brush with the trigger texture on it and assign it to a trigger_multiple and give it this bit of info

"target" "alarm"
"spawnflags" "1"

5.Ok nearly there, place a point_combat entity just inside of of the trigger_multiple brush (or at least make it so the monster will touch the trigger_multiple, heres the values for the point_combat

"targetname" "ala2"
"spawnflags" "1"

6.Somewhere near the alarm button place a target_speaker entity and make sure it has these values

"noise" "world/klaxon1.wav"
"spawnflags" "2"
"targetname" "alarm"

7.Thats it! all being well once you've compiled your map the monster should run towards the button and set of the alarm.

Heres the example map