Clueless,Don't waste time trying to use the percussion map to freeze stems up or down. Use the percussion map to set up MIDI entry, playback and individual note shapes of the percussion staff instruments. The best method is to use to get the stems going the way you want them is to use layers. (Shane described the method perfectly, but used the incorrect term voices, which is something different.) You can also use the Mass Mover, as Shane described, but the place to find "Freeze Stems Up (or Down)" is in the Mass Mover Menu > Utilities not the Edit menu.
Jim Coull