arcgis desktop - Why is "calculate geometry" grayed out when data frame properties shows map units are in meters not degrees?

I'm using ArcMap 10.0 and am trying to get the length of a polyline by adding a field to the attributes table and calculating geometry, but that option is grayed out. Someone suggested that it's because my map was in degrees but according to the properties for the data frame, both display and map units are in meters, which means I should be able to get the lengths of these lines, right?