spine-libgdx uses a project reference to libgdx so we don't have to check in the binaries for libgdx, which change quite often. If you don't run libgdx from source, then you need to right click the spine-libgdx project, Properties, Java Build Path, Projects, and delete the gdx project reference. Next you'll need to let the spine-libgdx know where you have libgdx (eg, add gdx.jar to Java Build Path, Libraries).
We'd love to have Gradle support for spine-libgdx, so you can just check a box during libgdx setup to include Spine support.
