If what your are trying to do is to assign the PWR net class you created to a trace, this is how is done.
You have to select the net class you what to use in net class membership on the right(green circled) and on the left choose the net you want to move to that net class member(blue circled).
Both:
for new tracks to get the clearance and width based on the netclass membership.
for changing already laid down tracks, i can assign a new netclass membership or choose a different width and double click on the track to change the value.