I'm also trying to do a network install of Linux Mint using PXE (my laptop doesn't have a CD drive), but without success. Works fine with other distros (eg: ubuntu, gentoo).
Here's what I did in my attempt to get it working:
Copy all files to my tftpboot directory
Add LinxuMint my pxelinux.cfg/default file
It loads the initrd and kernel, but after a few seconds I get kicked to a busybox prompt.. I could not find the preseed/ubuntu.seed, so I can't passe it in the APPEND statement. Also, I highly suspect that the system doesn't know how to get isolinux/filesystem.squash once PXE is done..
Hope that made a little bit of sense.