Here's a very handy tutorial for building in Blue Mars using the free and fairly easy-to-learn Google SketchUp program, written by Desmond "the Guvnah" Shang, owner of Blue Mars' steampunk city of Caledonia, a masterpiece of collaborative creation. Desmond's basic workflow goes like this:
a) Make some textures in Photoshop or something.
b) Draw a 3D object with the free Google SketchUp.
c) Texture it up in SketchUp.
d) Export it from SketchUp (essentially a 'save' option)
e) Import it using the Blue Mars Item Editor.
f) If you have leased a block of land, you can set out items on the land parcel (block) with the Blue Mars block editor.
g) Then you upload the land parcel (block) and it will be uploaded with the next city update.
... but even if you're not leasing a block of land at the moment, you can still get your content into the offline Item Editor. Read it all here.