If the New Entry and the Replace Entry buttons on the MPS Index Tagger floating panel are grayed out and not available, first confirm that you have used the InDesign Selection tool to select a single frame (or a single group). 

Next confirm that you have selected list items on the top of the panel from either or both the Mfg list and the Product list(s). The selected items will be displayed in the Selected box on the bottom left side of the MPS Index Tagger floating panel.

Before clicking the Replace Entry button, confirm that you have selected the desired entries. The Assigned box on the bottom right side of the MPS Index Tagger floating panel will display the entries that are currently assigned to the selected text frame.