To start basic addon development for Pokecube, First download the following Zip:

https://github.com/Thutmose/Pokecube/blob/1.12.x/BasicDev.zip

Unzip this to where you want your workspace to be, then run runSetup.bat. If you use IDEA or some other IDE, make the appropriate alteration to runSetup.bat.

Once the setup is finish running, open eclipse, and In eclipse, click File, then Import, then under General, select Existing Projects into Workspace. Click browse, and then find the directory you unzipped it to, and it should find the workspace.

From there, you can start making an addon for pokecube.

Here is an image based version of the above: Extract zip: Run Setup: Setup finished: Open Eclipse: File -> Import: General -> existing project: Browse -> Find where you extracted and ran the setup: Finish: Done: