If the cost of staging an ATLAS project is more than the award can pay, is it permissible to charge admission and/or accept donations to offset other costs?

Yes. The proposal should clearly indicate how much money is needed to complete the project as envisioned and, if other sources of funds are needed, where the additional funding will be generated. The focus of ATLAS is completion of projects, so reviewers should be able to see how the project can be finished as planned and with the resources available.