SELECT AddGeometryColumn ('<schema>','<table>','<your_new_geometry_column>',<spacial reference>,'<geometry type>',<dimension>);
example:
SELECT AddGeometryColumn ('gem','pop_vals_ita','the_geom',4326,'POINT',2);
to update existing geometry columns:
SELECT Populate_Geometry_Columns();
then check to see that you geometry_columns table is populated with information about each geom column you have.
If any of those commands fail you may need to explicitly specially enable you db with:
createlang -U geonode -W plpgsql $DB_NAME
psql -U geonode -W -f /usr/share/postgresql/8.4/contrib/postgis-1.5/postgis.sql $DB_NAME
psql -U geonode -W -f /usr/share/postgresql/8.4/contrib/postgis-1.5/spatial_ref_sys.sql $DB_NAME
Cheers, Ben