i find the best way to do it is to go to Account Settings -> Business Policies then select the postage option you want to set the postage rate tables on for and click "Edit." 

you should then get a screen which looks something like this.. (i can't draw circles with a mouse evidently). you'll want to then check the tickbox which says "Apply domestic postage rate tables." then you need to do this for any other postage policies you want to apply the postage rate tables to :]