I was having the same crashes on export from SketchUp (OSX 10.4.3, with both SKP4 and 5), but I have finally got Skp2Mxs export to work.
Following the directions to make the ~/.MacOSX folder places this folder in the Users directory, not the Root folder which I think is the problem. The environment.plist file is still pointing to Root, not your User.
Try the following, it worked for me.
1. Using TextEdit open the environment.plist file that you copied into the ~/.MacOSX folder;
2. On the line that says :
<string>/Applications/maxwell.app/Contents/MacOS</string>
Insert a ~ before the /Applications so that the line looks like :
<string>~/Applications/maxwell.app/Contents/MacOS</string>
3. Save and quit TextEdit.
4. Open up SketchUp and export to MXS.
Hopefully that helps.
Tim