To use filters on the grid panel you have to do the following:
add 'Ext.ux.grid.FiltersFeature' to the 'requires' property of your grid,
add filters as a feature to your grid panel:
features: [ { ftype: 'filters', local: true } ]
and finally, mark the columns you which to have the filter using the property 'filter':
{ xtype: 'gridcolumn', filter: { type: 'string' } ... }