The wall should be a func_brush, and the trigger should be a trigger_ff_script--which defines the volume in which you want the detpack to work.
Lua is a scripting language that controls all the map rules. You need one for your map to work. Create a text file, and give it the same name as your map, ending in .lua, and put it in the /maps folder.
Just put one line in the Lua file for starters:
Code:
IncludeScript("base_teamplay")
This gives you support for team spawns, ammo bags and more for up to four teams. All you have to do is place the entities in your map and give them the proper names.
See this reference.
So, if you put a trigger_ff_script in your map named
blue_detpack_trigger, it already knows to respond to blue detpacks. Then if you place a logic_relay entity called
blue_det_relay, the Lua script will cause that to trigger. Logic_relay is great because you can fire any kind of outputs from it: remove your wall, play a sound, create gibs, etc. If you are unfamiliar with Source's I/O system,
read up on it, it's really useful.