When building your own Burp Suite extension, it can occur that compiling your code and creating the jar works find, but that you get the exception java.lang.ClassNotFoundException when loading your extention in Burp Suite. This article shows the solution to your problem.
The reason is suite simple: Burp Suite expects to find your implementation in a Java package called "burp". Having the line package burp;
in your source files is not enough! You also have to take this into account when creating your jar file.