Many thanks for you work on ODK, an helpful tool for development organisations. I would also like to test ODK central in order to decide if my organisation should merge from our current setup (one aggregate server per country, 7 servers in total, each having multiples projects running). Among other challenges, we are looking for a way to grant read/export access to collected data on a project basis (for sharing with external partners, as well as limitation to data collector from the same mechanism. (could not find how to send a private request via the forum)
We started the sandbox when Central was in beta and installation instructions were evolving. Central is now production-ready and there are a variety of ways to get a server. For that reason, we retired the sandbox on January 31st.
If you still need to try Central, the easiest way forward is with ODK Cloud. ODK Cloud provides Central installs that are fully managed and supported by the core team. Learn more at https://getodk.org#odk-cloud