MySQL Workbench EER Diagram and ENUM data type
ENUM datatype was not supported in MySQL Workbench v5.x. I had the same problem with MWB v5.2. I just downloaded the latest 6.x (v6.1.4) and was able to create an ENUM which now appears in the Datatype dropdown menu of the table editor.

One word of caution regarding ENUMs, as noted in the MWB docs:


  Caution:
  Care must be taken when entering a default value for ENUM columns because a non-numeric default will not be automatically quoted. You must manually add single quote characters for the default value. Note that MySQL Workbench will not prevent you from entering the default value without the single quotation marks. If a non-numeric default value is entered without quotation marks, this will lead to errors. For example, if the model is reverse engineered, the script will contain unquoted default values for ENUM columns and will fail if an attempt is made to run the script on MySQL Server.