Hi there Rock!
I´ve been using MCedit pretty much since the beta/alpha releases of Minecraft.
If i understand correctly, you're trying to copy an existing schematic into a world.
The best way, you can do this, if the file is too big is the following:
1. Open MCedit (Hurrr)
2. Open a Level
3. Instead of selecting a .dat file in the open a level, open the schematic.
(
http://gyazo.com/0e77a17d9e23cf2f603fd0ea5039a63a)
4. split up the build how many parts you feel necesarry, 4 usually does the trick.
Just select a region and, then copy it (CTRL-C). Proceed with clicking
5. In the end, you will have 4. unsaved schematics in the top-right of your screen.
http://gyazo.com/b7a839088f51a371a62999d1338723d06. Proceed to save all of these files, just name them 1-4 for your convenience (Castlepart1, Castlepart2. etc.)
7. Import these files on your map now, one at a time
8. Save after each import!
9. Succes!
Hope this has helped
