This software is currently alpha! Be warned, there will be bugs. We'll love you if you try it anyway, and report any bugs you encounter in the issue tracker. Please mention the git commit that your version is built off, if you do not know what this means or have used the auto updater just see the title of the application. It will look something like f648d0f.
You can always get the latest version by using the auto updater, downloadable from here (you only have to download this once). For a technical summary of recent changes, see this.
If you wish to install from source, the following should get you set up (requires git, maven & jdk7):
git clone https://github.com/r04r/KSPModClient.git
cd KSPModClient
mvn clean compile assembly:single
java -jar target/KSPModManager*.jar
Steam launching is implemented. Simply go to 'Settings' and enable Steam Launch. If you don't have Steam, it will throw an error when you try to launch. If you do have Steam and it still won't launch, please post an issue!