domes come in three different kinds
floor level cheats
objects you place like a statue
rounded roof parts (added by one of the expansion packs, not sure if it was Open for Business or Seasons)
The Floor Level Cheat kind is kind of tedious and hard to get just right - one example of a tutorial can be found here:
http://mikeinside.modthesims2.com/building/curvedroof/index.htmlObject sorts of things you can download from a variety of places on the internet, you can see some examples by SimArchitect here:
http://www.modthesims2.com/member.php?u=231795The Maxis build mode thing ... well either you have it or you don't