The RETENTION parameter is only displayed on the Script tab if its value is set to a non-default value.
If the RETENTION parameter was never set and defaulted to its default value, "AUTO", the RETENTION value is not displayed.
The RETENTION value should always be displayed. From the script Options dialog, there is a Storage Objects tab that has a LOB parameters drop down. Available options include:
all LOB parameters
no LOB parameters
no Storage clause
tablespace only
When "all LOB parameters" is selected, it should show all of them - especially the RETENTION - its default value or otherwise.