1. Place MCEdit onto your dock so that you can easily get to it.
2. Right click on the icon then options > Show in Finder
(Mine is in my downloads, but I'm sure it doesn't matter)
3. Right click it in the new finder window and go to "Show Package Contents"
4. There should be something called "MCEdit-0.1....0_7-x etc. Click that so you see it's contents
5. Right click the thing labeled "MCEdit" and click "Show Package Contents" again.
6. Open the file labeled "Contents"
7. Open "Resources"
8. There should be a file called "Filters"
9. Download any filter and place it in that folder
10. You now have a new filter added to your MCEdit!
If you do not see the filters folder,
1: Head into MCEdit
2: In a world, make a selection
3: Press the Filter button
4: On the left of the screen appears a box with Filters in Blue
5: If you click on the button a window open.
6: Put all your filters here