That's like, the problem... The entity was removed from the game code because who the hell would use a laser with paintball...
If you want a tripwire, use a 1x1 unit brush, and a 1x1 unit trigger_multiple, and stick them over the top of each other. Make the brush trans33 and mist, and voila, you have a tripwire.
Edit: And if you wanted to hurt/kill someone, like a laser, you could replace the trigger_multiple with a trigger_hurt or a targeted killbox.