Slime is the only one with an MD3 not in Jeck's pack, you can get the rest of the characters' MD3s here. https://mb.srb2.org/showthread.php?t=44244
To install the MD3, do as such:
1. Merge the mdl folder with your own.
2. Open up the file known as "mdls.dat"in the root SRB2Kart folder with a notepad program.
3. Copy and paste the following snippit of code into a new line.

SLIMETERRARIA KDLC/SLIM.md3 3.0 0.0

4. Save mdls.dat and you're done! I wish all of my fans a whole hearted thank you for pumping me up to release these three characters with as much quality as I am! It means a lot to me!