Rather than launch via the command line every time, I use the following script to patch the --enable-extensions command line arguments into the application bundle itself:
#!/bin/sh
# Patches the Chromium application installation with command line arguments to enable extensions.
set -o errexit
app=${1:-/Applications/Chromium.app}
cat >$app/Contents/MacOS/Chromium.wrapper <<EOF
#!/bin/sh
exec $app/Contents/MacOS/Chromium --enable-extensions "\$@"
EOF
chmod 755 $app/Contents/MacOS/Chromium.wrapper
defaults write $app/Contents/Info CFBundleExecutable Chromium.wrapper[robg adds: To use the above, save it in Terminal text editor, make it executable (chmod a+x scriptname), and then run the script. You can find Chromium Mac builds here, and this hint explains how to automatically update to the latest build. I tested this script, and it seemed to work with the latest build of Chromium. Note that the script assumes Chromium is in /Applications; if you have it elsewhere, you'll need to modify the app=... line.]

