If I understand you right, you want the dimension to be red, so you can adjust the value and update the measurements of the element? The padlock button on the bottom left side of your smart dimension toolbar is probably not checked. This is what changes dimensions between Driving dimensions (red, locked, editable by user) and Driven dimensions (blue, unlocked, resultant dimensions only).
It is also possible that your constraints on other elements make it logically impossible to switch a dimension from driven to driving. In this case, you would have to select some other dimensions and uncheck the padlock to change them to driven, THEN go and make the target dimension driving.
I am having the same problem. Does anyone know how to fix it? I do not have the option to change the lock. It is greyed out.
You have probably inadvertantly toggled the "maintain relationships" button off. Make sure at the top of your ribbon bar the button I show here is ON -- (orange).