How do you add a bash script to your PATH variable using symlinks?

You actually don't need any symlinks, just edit your ~/.bashrc and add the following statement:

PATH=$PATH:$HOME/PocketMine

This avoids polluting your filesystem with unnecessary clutter like symlinks. If you are a csh/tcsh user rather than a bash user, then edit ~/.cshrc and add

set path = ( $path ~/PocketMine )

Personally I'd go one step further in organization. I'd create a ~/bin directory, and put your start script in there, perhaps with a more distinctive name like pmstart (it's not significant that it's a shell script, is it? Maybe someday you want to re-implement it in Python or something. .sh suffices on executables are usually a bad idea because you're exposing and hard-coding an implementation detail (the implementation language) that end-users don't care about, and in the process committing yourself to that implementation detail unnecessarily).

It's likely your $PATH already includes $HOME/bin, but if not you can add it similarly.

Finally, there's are historical conventions/best practices as to what things go into sbin directories and what things go into bin directories. sbin is usually reserved for tools for administrators, while bin is for general end-user utilities.

If you just want to have a shorter way to run ~/PocketMine/start.sh, you can make an alias for it. Add the following line in ~/.bashrc and call pm to run that script:

alias pm=~/PocketMine/start.sh

If you need to change to the script's directory first, make it a function:

function pm (
  cd ~/PocketMine && exec ./start.sh "$@"
)

If you want to be able to call this shortcut from another script and not just from an interactive shell, make the shortcut a script that you put in any of the directories in $PATH. The script needs to be an executable file (e.g. chmod a+rx ~/bin/pm) and contains

#!/bin/sh
cd ~/PocketMine && exec ./start.sh "$@"