In a number of activities on the AgilePoint Data Entities tab, it refers to the Record ID. Should I use the Data Entity ID field or the Record Display Name field?

Always use the ID field.  The reason of this is that the ID field contains a Globally Unique Identifier, or GUID.  The GUID is the industry standard for unique data.  When any of the AgilePoint Data Entities activities ask for the ID, it is so it can positively link what you are doing with one and only one record in the Data Entity.  For a detailed explanation of the difference between the ID and the Record Display Name fields, see the 10 minute, Class 6 Video 1 recording in the AgilePoint on-line training program.