Problem with including mate in equations!

Hello all,

I am trying to add an equation between a feature and a mate in an assembly so that when the mate value is changed the feature regenerates.

In my case I want to add an equation between angle of sweep and a distance mate. After I double click on the feature and then double on the dimension to be added an equation to, I select the distance mate but it's not adding to the equations window! Please help!

Regards

SolidworksMotion Studies