I need to create a helper indicator like you see in SC2 campaign all the time. In the images, there is a green one on where the Hatchery should be built and a red one on the Generator to be destroyed. I was looking at some of the campaign mission files, but I don't get how to make the function needed to display the helper model. Any help would be great.
Those specific effects are created by triggers using the action "create reticle effect" and "attach reticle effect to unit". Those essentially are "create model at point" actions, but they have the right models (those indicators you want) already setup as a combination of race and objective-kind (main objective, bonus objective etc) and are therefore easier to use.
The reticle itself is an actor. Create a variable of type actor and after you used the create reticle action you use the Set variable action to set your variable to Last Created Actor . To destroy the reticle, you have to use the action Send Actor Message. As message set Destroy and as actor the variable you just created.