You have to grab the slick2d source code (should be supplied in the slick2d download), and grab the .java files you want to edit. Then place them in your Workspace in the same folders.
1. Right click on your \src\ folder in your program
2. Click "New Package"
3. Name the new package org.newdawn.slick
3b. Repeat, make another package called org.newdawn.slick.tiled (Or make a \tiled\ folder in org.newdawn.slick)
4. Go into the Slick2d zip file you downloaded (Not just the jar, the file with everything in it, sourcecode, javadocs, etc)
5. Navigate to \src\org\newdawn\slick, drag and drop SpriteSheet.java into org.newdawn.slick
6. Navigate to \src\org\newdawn\slick\tiled and drag and drop Layer.java and TiledMap.java into your org.newdawn.slick.tiled folder
The alternative method would be to just drag+drop the entire Slick2d source code into org.newdawn.slick and delete the reference library completely inside your project, but then you'd be recompiling the entire slick2d source code every time you export the game, really wont hurt anything but why bother? One benefit to this is once your game is "complete" you could selectively remove parts of Slick2d you don't use in your game. But really, that'd be silly. Slick isnt that big.