Extract Source Code

If you mean extracting the mod:

1. Next to the play button, (on the right of it) click that and fill in
2. Done :D

If you mean extract the source code:

1. Next to 'TOOLS' there is 'CODE'. Go there
2. On the left there is a list of all the mods you created. Double-Click on one.
3. Click 'Edit without locking' when the popup shows.
4. Copy and paste the code -- From here you can either send the code with pastebin. To send as a file, continue.
5. Put the code into a text editing software (Notepad, Notepad++..)
6. Click Save as (Follow the next steps very carefully)
7. Under the place where you can write your file name, click on the file type dropdown
8. Select 'All types (*.*)'
9. Save as anything.java

Hope this helps.

Cyclip gave you a very accurate description :)