How do I convert an .lxf file to an LDraw file for bricksmith on Mac?
If you have a LEGO Digital Designer file (.lxf) and want to convert that to an LDraw compatible format so that LDraw-based tools such as Bricksmith can read it, your best bet is to use LDD's Export functionality to save your file as a .ldr file. The full details on this procedure are located here. 

This post covers both LDraw to LDD as well as LDD to LDraw conversion. 

Basically, there are 3 key steps:


Make sure LDD has an updated ldraw.xml file. This resets part origins, colors and part numbers.
Use the 'Export model' (CTRL-E) functionality in LDD to save your model (after you have saved it as a .lxf first). Make sure to pick the right file-type, and heed the warnings from the article about making sure you don't accidentally overwrite your .lxf.
Open the exported model in your LDraw tool of choice.