These are instructions for Bukkit HERE.

Download Obole and extract the Obole.jar file into your "plugins/" folder
Give out permissions, as per the Obole wiki (give obole.* for all).
Run your server and Obole is installed.
NOW edit configuration !

Web files

Download ipn.php and config.php
Upload webfiles to your webserver and edit conf.php
Log into Paypal and go to Profile > My Account > More Options > My Selling Tools > Instant Payment Notifications and enter the URL to the ipn.php
file included in the web folder.
Edit your paypal button (on your donate page ) and add this line

<input type="text" size="60" name="custom" value="Minecraft username">

Obole work with the name="custom" for check minecraft username, don't remove it.