Try running selecting the JAR file with right click and select run with: And Java should be an option if you installed it properly.
Obviously Mamba knows what he is talking about, but the Jar file is the only thing that works for me. And, like I noted, he should link to the specific Java version that works. All his mod does is drop you off on the Java website, which is confusing as hell for people that have zero programing knowledge or understanding.
I guarantee more people have issues with installing Java then they do the mod.
I don’t use GD-Stash anymore because Item Assistant is just better for inventory management and it works automatically without java. It also just takes the items out of your stash instantly and gives you a notification. You may want to try that unless you want the cheat functions of GD-Stash.