To get a free trial of MATLAB Builder JA, just go to the product page, and click the link on the right that says "Product Trial". You should be able to immediately download a short-term (about 15 days) trial without needing any involvement from a human at MathWorks.
However, if you call sales on the phone, I'm sure they can arrange for a trial pretty quickly as well.
Bear in mind, though, that the .jar files output by trial copies of MATLAB Builder JA expire at the same time as the end of the trial period (the same is true of the components produced by MATLAB Compiler, and the other Builder products).