It appears that the Employee type does not exist yet.
In order to add a field you need to have an existing type present. So in your example you would issue two commands:
The second command will add the doj field to the Employee type. In case you want to add that field to another (already existing) type you would need to use the --class attribute:
entity --name ~.domain.Employee
field date --type java.util.Date --fieldName doj
The trick is to use the tab key as much as possible when typing your commands. That will reveal which attributes are available on commands and which attributes are mandatory.
field date --type java.util.Date --fieldName doj --class ~.domain.MyOtherType
So in short, when Roo tells you that "Java source code details unavailable for type ..." that means you will probably need to create that type first using the 'entity' command or the 'class' command.