Click on the page in the Hierarchy and then go the Pagination tab of the Object Palette, the top option is Place - the default is Top of Next Page - click the dropdown and go to On Page and select the name of the Master Page you want.
Click on the page in the Hierarchy and then go the Pagination tab of the Object Palette, the top option is Place - the default is Top of Next Page - click the dropdown and go to On Page and select the name of the Master Page you want.