To add your weights to your workout you can do this during the workout by clicking the dumbbell icon on the right hand side. This can't be modified before or after the workout.
I can't see a dumbbell icon in my workout.
If you can't see a dumbbell icon after starting your workout, weight tracking is not currently enabled for this type of workout.
I can't see my weights as added?
Each workout needs to be marked as ticked to ensure the weights are logged.