Hey matt, im not sure i follow you on this. Do you have a part drawn and you are trying to dimension it in a drawing file? Or are you trying to add this dimension in a sketch?
The approach to modeling something in SW vs Acad is different. The dimensions you add in a sketch are the parameters that define the sketch(very important that they be defined). Unlike Acad. Can you maybe upload a zip file or put a screen shot or what you are trying to do.