Official Tutorial says this, I had try everything similar and there is no way to show up the fx and equation option, please help as getting crazy on this.

I'm using 3Dexperience online on xdesign app.

Thanks for you help in advance.

Double left click on the offset dimension to modify.

1. Select parameter A from the Design Tree. 
2. Select A(1.5mm) in the Equation dialogue box.
3. Click Ok.