I assume you're trying to use TextMate as your default editor. This error is telling you that 'mate', the command line tool to run TextMate, is not on the path.
You must add it to your system path. First you must locate your mate program; here's where mine is:
Nick@Macintosh-3 ~/Desktop/Programming/Minesweeper master$ which mate
/usr/bin/mate
Nick@Macintosh-3 ~/Desktop/Programming/Minesweeper master$ ls -la /usr/bin/mate
lrwxr-xr-x 1 root wheel 50 Mar 30 2009 /usr/bin/mate -> /Applications/TextMate.app/Contents/Resources/mate
So it's in /Applications/TextMate.app/Contents
Make a simlink to /usr/bin/mate:
ln -s /Applications/TextMate.app/Contents/mate /usr/bin/mate
Now when you type mate at the command line, you should launch a new TextMate instance.