trying to make a dimension driving

I am in sketch mode  I am having difficulty creating a driving dimension

When  I click on  on a sketch and Right  click on the dimension   a menu pops up but it does not show  the sketch section where  I can select drivin , I have spent too much time trying to figure this out and have not been sucessful hope someone can help .

this is for a creating a spring to be used in an assembly

hope someone can help 

SolidworksSketching