The structure of the "Bricksmith Parts.plist" file is 

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Part Catalog</key>
	<dict>
		<key>Animal</key>
		<array>
			<dict>
				<key>Part Name</key>
				<string>Animal Horse Hitching</string>
				<key>Part Number</key>
				<string>2397.dat</string>
			</dict>
			<dict>
				<key>Part Name</key>
				<string>Animal Bird</string>
				<key>Part Number</key>
				<string>2546.dat</string>
			</dict>
...
		</array>
		<key>Antenna</key>
		<array>
			<dict>
				<key>Part Name</key>
				<string>Antenna  6H</string>
				<key>Part Number</key>
				<string>104.dat</string>
			</dict>
...
		</array>
	</dict>
	<key>Part List</key>
	<dict>
		<key>1-16cyli.dat</key>
		<dict>
			<key>Part Name</key>
			<string>Cylinder 0.0625</string>
			<key>Part Number</key>
			<string>1-16cyli.dat</string>
		</dict>
		<key>1-16edge.dat</key>
		<dict>
			<key>Part Name</key>
			<string>Circle 0.0625</string>
			<key>Part Number</key>
			<string>1-16edge.dat</string>
		</dict>
...
	</dict>
</dict>
</plist>

To add recognition of a part, you need to follow these steps:

1) go to ldraw.org and find the Parts Tracker. 
2) locate the part you want, and download its .dat file.
3) put the .dat file in LDraw/PARTS/ in the Bricksmith folder.
4) Edit a copy of the Bricksmith Parts.plist file, found in the LDraw folder in the Bricksmith folder. Make sure to use a well-behaved text editor, like TextWrangler, and not a word processor, which might put extra garbage in the file. You need to add two entries for each part. 
4a) The first  is in the Part Catalog, in the appropriate section. The Part Catalog has a subsection, identified by a key entry, for each type of part. Your new part will appear in the parts catalog drawer in Bricksmith in whichever section you put it, but it doesn't seem to matter where within that section you put it. Make your new entry match the surrounding ones, except of course with the part number and part name of the part you're adding.
4b) The second place is in the Part List. Again, where you put it in the Part List doesn't seem to matter, but i prefer to put it in the same order as i did in the Part Catalog--right before or after the same part. Again, mimic one of the existing entries, but substitute the information for your new part. Make sure to insert the part name in both locations: the key that identifies the entry, and the string within that entry.
5) Put the original Bricksmith Parts.plist file somewhere safe, just in case you screwed this up, or want to go back to it.
6) Put your edited Bricksmith Parts.plist file in place of the original. 
7) Start up Bricksmith, and you should find your new part both when browsing, and when searching, and it should show up in the part browser window and be insertable. If it's not in any of these places, you did something wrong.

....
Or, as it turns out, BrickSmith has vastly simplified the whole process with the latest version. All you need to do is download and  put the .dat file in the appropriate place, then go into BrickSmith's preferences and tell it to rescan.
For these models, I've added the following parts (and supporting subparts, with their appropriate locations): 
48729.dat:	Bar  1.5L with Clip
52107.dat:	Brick  1 x  2 with Studs on Sides
53989.dat:	Minifig Mechanical Arm with Clip and Rod Hole
49668.dat:	Plate  1 x  1 with Tooth
54200.dat:	Slope Brick 45  1 x 1 x 2/3
4697b.dat:	Technic Pneumatic T Piece - Type II
61678.dat:	Slope Brick Curved  4 x  1
61409.dat:	Slope Brick 17  2 x  1 x  2/3 Grille
53451.dat:	Minifig Accessory Helmet Viking Horn
60478.dat:	Plate  1 x  2 with Handle on End
57585.dat:	Technic Bush with Three Axles
	p/axl2hol2.dat:	Technic Axle Hole Reduced Side Edges
	p/axl2hole.dat:	Technic Axle Hole Reduced
60470.dat:	Plate  1 x  2 with 2 Clips Horizontal
	p/box4-2p.dat:	Box with 4 Faces without 2 Parallel Edges
45590.dat:	Technic Axle Joiner Double Flexible
	parts/4198367.dat:	_Technic Axle Joiner Double Flexible Rubber_Black