MassEffect accesses the Salesforce meta data (fields and object data) differently than the other DemandTools Modules. MassEffect creates a file of the meta data which is stored directly on the users PC in the DemandToolsData\Cache directory. That file is automatically created the first time a user accesses the module for a particular instance and automatically refreshed when a user upgrades to a newer version of DemandTools.
DemandTools references that file to display the list of objects and fields in the user interface. When users add new objects to Salesforce, the Update Newly Added Objects button (lower right) should be selected to manually refresh the meta data file. This will ensure that all editable objects are made available in MassEffect.