Quick and dirty:

download the .po file
edit in bbedit
save

get the gettext tools from:

http://download.videolan.org/pub/videolan/devtools/gettext-macosx.tar.gz

`
mkdir -p /usr/local/lib
cp gettext* /usr/local/lib/
ln -s /usr/local/lib/msgfmt /usr/sbin/msgfmt
/usr/local/lib/libintl.a</p>

msgfmt --statistics -o nl_NL_merlijn.mo /Users/merlijn/Desktop/NL_nl.po
` upload the mo file, edit wp-config.inc done