Custom Entities; how to place them?

[Theforgot3n1]

Hello ya'll!

My name is Theforgot3n1. I'm wondering how I can add custom entities to my map. So far I've made the entity, which consists of combined stuff from other entities, and I've put it in "custom_stories" > "(name of my custom story)" > "entities" > "door (the category)" > "(entity here)".
But as I navigate to it in my level editor, and create it somewhere on my map, the next time I load the map it gives me an error message and deletes the custom entity. Is it possible to have custom entities in CS? I have custom particles, sounds and such, but it malfunctions with entities.
I've played my map with the entity, and it worked. It's only when I reboot my computer and try to load the CS that this happens.
Thank you!