Maybe Spring team already corrected this bug - I'm not sure about your environment and tools. You can see the correct variable name in the database.properties file. To complete the IBM tuturial mentioned above, I used STS 2.9.2, Ubuntu 12.04, MongoDB locally installed.
STS 3.1.0, Windows 7, Mongodb locally installed.
The dbproperties file says the variable is mongo.name.
The problem may relate to the bug in STS. Roo projects doesn't work when created within STS. You should use 2.9.2 or 3.2.0M2, or create the project in cmd mode, without STS.