Join the Development
Helping to make the Mathematica plugin better can happen in various ways and only one of them requires that you write code. More important than actively implementing features, is carefully testing the plugin to reveal bugs and unwanted behavior. For you, this means nothing more than using the plugin in all possible ways, try all features in all places you can think of, to see, whether it works like you expected it to work. If it doesn't, then..
...help us by reporting bugs or suggesting features
NOTE: We moved the bug database to GitHub as it is more convenient to have code and issues at one place!
If you find bugs, experience unusual behavior or have feature requests you can simply create a new issue in our GitHub database. Just give the issue a meaningful Summery title and add a description which allows us to reproduce the error. You can use all kinds of Markdown formatting e.g. to insert code or upload screen-shots.
You want to build and change the plugin yourself?
No problem! The Mathematica IntelliJ IDEA plugin is open-source and hosted in the Mathematica-IntelliJ-Plug-in GitHub repository. Besides the fact, that it is completely free and you never have to pay a cent for it, this further means you can actively help to make it better and turn IDEA into the one Mathematica IDE you have always dreamed of.
The plugin is written in Java using the Plugin API of IDEA. If you want to compile the code on your machine you first need to set up your development environment correctly. Besides having a Java SDK and a working Community Edition of IDEA some other steps are required. To ease this setup I prepared a very detailed article and two screencasts.
Here are some important resources that are very useful