First of all, you'll need to get joe installed. You can do so by opening a command prompt and typing
Enter your root password and confirm that you'd like to install joe. Afterwards, download the .jsf file from Google code.
Place it at the syntax subdirectory of joe. For me, it's located at /etc/joe/syntax and you can do so by
sudo mv lsl.jsf /etc/joe/syntax
Then copy the .joerc file to your home directory in order to make a small change to it. Type
sudo cp /etc/joe/.joerc ~
Edit the file and add the following lines to it's bottom:
Congratz! You just successfully set up your joe to highlight LSL syntax. Just save any lsl script as NAME.lsl and open it with joe.