Ok. The only problem is that shen I save a mod, it has an error that pops up. I saved one mod, and all it was was just a chest in the temple district with daedric items i edited. But, this is what happens now:
1. I open the construction set
2. I load all the data, sometimes the chest mod, and sometimes the normal Shivering isles and Knights of the nine
3. I use any district of the imperial city (and other cities) to put creatures in them out of nowhere.
4. After all is in place, I click save, and under Data i click save again.
5. The error report pops up saying an error has been detected and the program must close. Then the whole send-don't send option is at the bottom.
6. It saves, but as an invalid file so i can't use it. I've tried re-installing both oblivion and the construction set.

Hello ConstuctionSet Forum: http://www.gamesas.com/index.php?/forum/24-construction-set/.

Short answer, though. You can't load another ESP file when creating one. It will always crash.

There is a work-around if necessary called "esp mastering". Do a search in the CS forum.