Trying to add Time Intelligence and hitting an error...

I'm setting up a new cube, and trying to add time intelligence. The IDE doesn't give me the option, instead showing a warning message:

A time dimension is required to enable this functionality. Ensure that you have a dimension of type Time, that contains at least one hierarchy with a level flagged as a time period.

Now, as far as I know, I have a dimension of type Time and it contains a hierarchy of levels with Types that are time perionds (year/quarter/month/day), so I'm a bit puzzled by this message.

My Time dimension is represented by a physical date/month/quarter/year hierarchy in the relational database, and I have created a dimension hierarchy that matches. I've got attributes in my time dimension with types Days, DayOfMonth, MonthOfQuarter, MonthOfYear, QuarterOfYear and Years. The Type of the dimension is set to Time.

What am I missing