<edit>The path in step 10. has been corrected following feedback from makaroni and belanger below</edit>

Here's my take on how to install and configure MediaBox from svn
1. install subversion from the Maemo Extras Repository
2. Open an Xterm. You should have a "~ $" prompt which is the users home directory (/home/user) you can confirm this with the command: pwd
3. Enter command: mkdir mediabox
4. Enter command: cd mediabox
5. Establish a connection to the internet
6. Enter command: svn checkout https://garage.maemo.org/svn/mediabox
7. Install Personal-launch from the Maemo Extras Repository
8. From the home page menu select Applet settings > Personal Launcher
9. As the name of the launcher entry enter: MediaBox
10. As the command to execute enter: python /home/user/mediabox/mediabox/trunk/MediaBox.py
11. For Icon to display, click the "Select from List" button, scroll down a little and pick something suitable.
12. click OK until the all the dialogs are closed.

You should now have an icon on the home page that will launch the latest mediabox. If nothing happens open the MediaBox entry in Personal-Launcher again and tick the "Run in terminal?" box. This should give you a clue to the problem.

To get the latest updates:
13. Open an Xterm
14. Enter command: cd mediabox/mediabox
15. Establish a connection to the internet
16. Enter command: svn up

These commands can be incorporated into Personal-launcher as:

Name of launcher entry: Mediabox update
Command to execute: (cd /home/user/mediabox/mediabox ; pwd ; svn up)
Icon to dispay: pick some thing suitable
Run in terminal?: checked.

If I've got anything wrong let me know.