MolecularWorkbench 100.0 for Mac was available to download from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The actual developer of this free software for Mac is The Concord Consortium Inc.
We recommend checking the downloaded files with any free antivirus. The application is categorized as Education Tools. The following version: 2.0 is the most frequently downloaded one by the program users. The unique ID for this app's bundle is org.concord.modeler.ModelerLauncher.
From the developer:
The Molecular Workbench™ (MW) software is a a versatile platform for science education. First, it is an open-ended modeling tool for designing and conducting powerful simulations across science and engineering. Second, it provides an authoring system for instructional designers to create and publish simulation-based curriculum materials.
You may want to check out more software for Mac, such as LabAssistant, Calcline or Schematics Plus, which might be related to MolecularWorkbench.