Sanicball is discontinued and I will not be accepting any pull requests. However if you have trouble understanding parts the project structure or code you can message me directly (viciousbadger@protonmail.com), I'll try to help out.
The project is to be opened with Unity3D. It works best with version 5.3.5f1 of the editor (Available here: https://unity3d.com/get-unity/download/archive). If you use a newer version, Unity will attempt to upgrade the project, which will most likely leave a ton of bugs for you to fix first.
Also check out https://github.com/BK-TN/SanicballServerList, which lets you host a server list for the game.